adds credential documentation

[m00sey]

adds a GitHub pages site describing credentials, their attributes and relationships.

[codecov]

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Additional details and impacted files

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[m00sey]

you can run bundle install and bundle exec jekyll from docs serve to preview the site

[kentbull]

Other than the "The "vLEI Credential Ecosystem - Dependencies and Schema Relationships" diagram this PR looks good to go. I found that diagram hard to follow and would create it in LucidChart since Mermaid seems to be struggling with the complexity of the diagram. I like the content there, just not how Mermaid is representing it.

[m00sey]

LucidChart is proprietary and has a soft paywall (sign up) so I do not think its use is appropriate for OSS.

There is an additional formatter for Mermaid (ELK) which works fine in the playground but it doesn't seem to work via Jekyll...

I've added SVGs of each diagram, rendered using ELK and links to them all on the index.