feat: add templates section

docs/templates/_category_.json

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+{
+  "label": "Templates",
+  "position": 18
+}

docs/templates/template-variables.md

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+---
+sidebar_position: 4
+---
+
+# Template Variables
+
+[[API Docs](/api/template-variables)]
+[[SDK](https://www.npmjs.com/package/@epilot/template-variables-client)]
+
+:::note
+This article is missing content (TODO)
+:::
+
+The Template Variables API provides variable discovery and substitution email and document templates using [Handlebars](https://handlebarsjs.com/).
+
+## Template Variables API
+
+This API is called to both discover available variables as well as execute the variable substitution using handlebars.
+
+Each time an email or document template is used, the Template Variable API is called with the appropriate standardised parameters.
+
+The Template Variable API uses the Entity API and others to fetch the correct values for each variable when compiling the template.
+
+## Variable Picker
+
+We provide a picker UI for users to search and explore available variables.
+
+![Variable Picker UI](../../static/img/templates/variable-picker.png)
+
+## Variable Builder
+
+Custom variables can be created using the Variable Builder under Configuration > Templates > Variable Builder, or by going directly to [https://portal.epilot.cloud/app/variable-builder](https://portal.epilot.cloud/app/variable-builder)
+
+![Variable Builder UI](../../static/img/templates/variable-builder.png)
+
+You can create the following types of variables:
+
+### Custom Variables
+
+![Custom Variables](../../static/img/templates/custom-variables.png)
+
+Custom variables can be composed of any text, and, make use of any of the [Handlebars Helpers](#custom-handlebars-helpers).
+
+To use a custom variable on your document template, use this syntax:
+
+```handlebars
+{{custom_variable_name}}
+```
+
+**Note:** The handlebars helpers can be directly used on document templates. Custom variables are helpful to contain and reuse complex logics or data.
+
+### Order Table Variable
+
+![Order Table Variable](../../static/img/templates/order-table.png)
+
+Order tables are used to display a table of items of a given [Order](https://docs.epilot.io/docs/pricing/orders).
+
+They are extremely customizable and can also make use of [Custom Variables](#custom-variables) or [Handlebars Helpers](#custom-handlebars-helpers).
+
+![Order Table Column Config](../../static/img/templates/order-table-2.png)
+![Order Table Column Config 2](../../static/img/templates/order-table-3.png)
+
+For example, customizing it's columns order, style, removing or adding columns, etc.
+
+![Order Table Column Config 3](../../static/img/templates/order-table-4.png)
+
+Or even completely changing how the table should be rendered, adding a custom header, footer, etc.
+
+To use a custom order table variable on your document template, use this syntax:
+
+```handlebars
+{{~~custom_table_key}}
+```
+
+## Custom Handlebars Helpers
+
+These are handlebars helpers you can use to further customize your templates.
+
+### Standard Handlebars Helpers
+
+Registers utility helpers from the `handlebars-helpers` library for categories like `math`, `number`, `date`, `comparison`, `match`, `array`, `regex`, `collection`, `object`, `string`, `html`, `markdown`, `url`, and `moment`. Examples include arithmetic operations (`{{add 1 2}}` → `3`), number formatting (`{{formatNumber 1000}}`), date manipulation, conditional logic (`{{ifEq a b}}`), array operations (`{{first arr}}`), and string manipulation (`{{uppercase str}}`).
+
+```handlebars
+{{add 1 2}}
+```
+
+### formatAddress
+
+This helper formats an address object into a string using `buildFullAddress`. Returns an empty string if the input is not a valid address object.
+
+```handlebars
+{{formatAddress 'billing_address.0'}}
+```
+
+### calculateColspan
+
+Calculates the colspan for table cells based on the table configuration. Starts with the total number of columns minus 3, adjusts for disabled columns, the presence of an `amount_tax` column, and whether `net_total` is enabled in the footer. Ensures the result is non-negative. Used on [Order Table Variables](#order-table-variable).
+
+```handlebars
+{{calculateColspan table_config}}
+```
+
+### calculatePeriodColspan
+
+Determines the colspan for period-related table columns. Returns 2 if there are more than 3 enabled columns, 1 if exactly 3 enabled columns and `net_total` is enabled in the footer, or 1 for fewer than 3 enabled columns. Used on [Order Table Variables](#order-table-variable).
+
+```handlebars
+{{calculatePeriodColspan table_config}}
+```
+
+### calculateSummaryColspan
+
+Calculates the colspan for summary sections by counting the number of enabled and draggable columns in the table header. Returns 0 if no columns are defined. Used on [Order Table Variables](#order-table-variable).
+
+```handlebars
+{{calculateSummaryColspan table_config}}
+```
+
+### isColumnEnabled
+
+Checks if a specific column is enabled in the table configuration by matching the column ID. Returns `true` if the column exists and is enabled, `false` otherwise. Used on [Order Table Variables](#order-table-variable).
+
+```handlebars
+{{isColumnEnabled table_config 'price'}}
+```
+
+### shouldDisplayDetails
+
+Determines if detailed information for a specific column should be displayed based on the `showDetails` property of the column in the table configuration. Returns `false` if the column is not found or `showDetails` is not set. Used on [Order Table Variables](#order-table-variable).
+
+```handlebars
+{{shouldDisplayDetails table_config 'description'}}
+```
+
+### isSummaryVisible
+
+Checks if the summary section is visible by determining if there is at least one enabled column that is not draggable in the table configuration. Used on [Order Table Variables](#order-table-variable).
+
+```handlebars
+{{isSummaryVisible table_config}}
+```
+
+### isExternalFeesMetadataVisible
+
+Determines if external fees metadata is visible based on the `enable` property of `external_fees_metadata` in the table footer configuration. Used on [Order Table Variables](#order-table-variable).
+
+```handlebars
+{{isExternalFeesMetadataVisible table_config}}
+```
+
+### gt
+
+Compares two numbers and returns `true` if the first is greater than the second.
+
+```handlebars
+{{gt 5 3}}
+```
+
+### lt
+
+Compares two numbers and returns `true` if the first is less than the second.
+
+```handlebars
+{{lt 3 5}}
+```
+
+### eq
+
+Compares two numbers and returns `true` if they are equal.
+
+```handlebars
+{{eq 5 5}}
+```
+
+### blockHelperMissing
+
+Returns an empty string when a block helper is missing, ensuring no output is rendered.
+
+```handlebars
+{{#missingBlock}}content{{/missingBlock}}
+```
+
+### helperMissing
+
+Returns an empty string when a helper is not found, preventing errors in the template.
+
+### makeStyle
+
+Converts a table configuration object into a CSS style string by mapping non-object properties to `key: value` pairs, joined with semicolons. Ignores object values and falsy values. Used on [Order Table Variables](#order-table-variable).
+
+```handlebars
+{{makeStyle table_config.style}}
+```
+
+### <schema>.<property>
+
+Dynamically registered based on schema attributes (e.g., `main.email`, `contact.name`). Retrieves a specific property value from the context data, prioritizing items tagged as "primary" or the first item in an array. If the property is an address identifier (e.g., `street`, `city`), it formats it as a full address using `buildFullAddress`. Supports nested attributes for repeatable or relational schemas.
+
+```handlebars
+{{main.email}}
+```
+
+### email
+
+Retrieves email data from the context, prioritizing `email` or `billing_email` identifiers. If the data is an array, it takes the first element. Returns the matched value or an empty string if not found.
+
+```handlebars
+{{email "contact"}}
+```
+
+### billing_email
+
+Retrieves billing email data from the context, prioritizing `billing_email` or `email` identifiers. If the data is an array, it takes the first element. Returns the matched value or an empty string if not found.
+
+```handlebars
+{{billing_email "contact"}}
+```
+
+### phone
+
+Retrieves phone data from the context, prioritizing `phone` or `billing_phone` identifiers. If the data is an array, it takes the first element. Returns the matched value or an empty string if not found.
+
+```handlebars
+{{phone "contact"}}
+```
+
+### billing_phone
+
+Retrieves billing phone data from the context, prioritizing `billing_phone` or `phone` identifiers. If the data is an array, it takes the first element. Returns the matched value or an empty string if not found.
+
+```handlebars
+{{billing_phone "contact"}}
+```
+
+### address
+
+Formats an address from the context data (e.g., `this`, `this.order`, `this.contact`) by searching for addresses with `billing` or `primary` tags. Uses `buildFullAddress` to format the address. Falls back to the first address if no tagged address is found.
+
+```handlebars
+{{address}}
+```
+
+### billing_address
+
+Formats an address from the context data by searching for addresses with `billing`, `shipping`, or `primary` tags. Uses `buildFullAddress` to format the address. Falls back to the first address if no tagged address is found.
+
+```handlebars
+{{billing_address}}
+```
+
+### shipping_address
+
+Formats an address from the context data by searching for addresses with `shipping`, `billing`, or `primary` tags. Uses `buildFullAddress` to format the address. Falls back to the first address if no tagged address is found.
+
+```handlebars
+{{shipping_address}}
+```
+
+### delivery_address
+
+Formats an address from the context data by searching for addresses with `delivery` or `primary` tags. Uses `buildFullAddress` to format the address. Falls back to the first delivery address if no tagged address is found.
+
+```handlebars
+{{delivery_address}}
+```
+
+### additional_address
+
+Formats an address from the context data by searching for addresses with `primary` tags. Uses `buildFullAddress` to format the address.
+
+```handlebars
+{{additional_address}}
+```
+
+### withTag
+
+Retrieves a value from an array of items based on a specified tag (defaults to `primary`) and optional attribute. Uses `getValueByTag` to find the matching item and return the attribute value or a formatted address.
+
+```handlebars
+{{withTag items tag="primary" attribute="email"}}
+```
+
+### yn
+
+Converts a boolean-like value (using the `yn` library) to a translated "Yes" or "No" string (via `i18n.t`). Optionally returns custom `success` or `failure` values if provided.
+
+```handlebars
+{{yn true}}
+```
+
+### xif
+
+Returns an "x" if the input evaluates to true (via `yn`), otherwise an empty string.
+
+```handlebars
+{{xif true}}
+```
+
+### customOrderTableVariable
+
+Renders a [Order Table Variable](#order-table-variable).
+
+```handlebars
+{{~~custom_table_key}}
+```
+
+### formatDateTime
+
+Formats a date/time string using `formatDateTimeIfPossible` with a specified pattern (defaults to `JODA_SHORT_DATE_TIME_FORMAT`).
+
+```handlebars
+{{formatDateTime "2025-05-13" "yyyy-MM-dd HH:mm"}}
+```
+
+### formatDate
+
+Formats a date string using `formatDateTimeIfPossible` with a specified pattern (defaults to `JODA_DATE_FORMAT`).
+
+```handlebars
+{{formatDate "2025-05-13" "yyyy-MM-dd"}}
+```
+
+### dateMath
+
+Performs date arithmetic (e.g., adding/subtracting days) using `calculateDate`. Accepts parameters like `inputDate`, `expression`, `inputFormat`, and `format`. Logs errors if processing fails.
+
+```handlebars
+{{dateMath "2025-05-13" "+1d"}}
+```
+
+### padStart
+
+Pads a string to a target length with a specified pad string (defaults to space) using `padStartHelper`.
+
+```handlebars
+{{padStart "5" 3 "0"}}
+```
+
+### generateJourneyLink
+
+Generates a journey link based on provided options using `generateJourneyLink`. Logs errors if processing fails.
+
+The helper accepts space separated key-value pairs as arguments (example: param1=value1), which end up in options.hash
+
+It requires journey_id to be passed as a parameter and can have the following manually parsed parameters:
+
+- custom_url: Custom domain to be used in the URL
+- expires_in: Expiration time of the token ([ms](https://github.com/vercel/ms))
+- nonce: Boolean to add a nonce to the payload
+
+The helper will add the initial_submission_id to the payload if it's available in the context
+
+```handlebars
+{{generateJourneyLink hash.journeyId="123"}}
+```
+
+### asCurrency
+
+Uses [@epilot/pricing](https://github.com/epilot-dev/pricing) to format the amount using `formatAmount` or `formatAmountFromString`. Accepts `currency` (defaults to `DEFAULT_CURRENCY`), `locale` (defaults to `de`), and `displayZeroAmount` (defaults to `false`). Returns an empty string for invalid or zero amounts unless `displayZeroAmount` is true.
+
+```handlebars
+{{asCurrency 100.50 "EUR"}}
+```