Create upgrade guide for Plone 6.2 backend.

[mauritsvanrees]

For technical review, I tag @davisagli @gforcada @petschki.

I have fixed most Vale feedback, except these, which I don't know how to write better:

venv/bin/vale --no-wrap  docs/backend/upgrading/version-specific-migration/upgrade-to-62.md

 docs/backend/upgrading/version-specific-migration/upgrade-to-62.md
 33:11  suggestion  'is deprecated' looks like passive voice.    Microsoft.Passive         
 33:44  suggestion  'be removed' looks like passive voice.       Microsoft.Passive         
 62:29  warning     Avoid using acronyms in a title or heading.  Microsoft.HeadingAcronyms 

That last one is for "Classic UI" in the last header, I think.

---

📚 Documentation preview 📚: https://plone6--2002.org.readthedocs.build/

Specifically this page: /backend/upgrading/version-specific-migration/upgrade-to-62.

[stevepiercy]

You can ignore Vale warnings and suggestions, but errors should get a look. As part of my routine maintenance, I try to address the errors and reduce them. There's a long way to go yet, so the only thing I really care about for now is to not introduce new errors. Often it turns out to be a candidate for a new term in the glossary, and sometimes it's just some bad MyST markup.

I'll do a thorough editorial review in the next day or so, after the smart people do a technical review. Thanks for the PR!

[gforcada]

Nice write up!

[petschki]

thanks @mauritsvanrees ... currently there is nothing more to say for the template movements. Progress is tracked here plone/Products.CMFPlone#3953

[stevepiercy]

Minor tweaks, and a question and suggestion for reference to Python docs. I'm not sure if I got it right. Thanks for your work on this. It's really good!

[stevepiercy]

We should include references to the following terms.

• Native namespace packages
• PEP 420 – Implicit Namespace Packages

If I read all that correctly, a native namespace is a subcategory of implicit namespaces. Is that right?

If so, then I'd make the following suggestion.

Suggested change

	There are three styles or techniques of namespaces.
	For Plone only two are important: `pkg_resources` and native (sometimes referred to as implicit).
	The third style is `pkgutil`, but Plone has never used it.
	There are three styles or techniques of namespaces.
	For Plone, only two are important: `pkg_resources` and native, the latter of which is a subcategory of implicit namespace packages where you omit the `__init.py__` file from the namespace package directory.
	The third style is `pkgutil`, but Plone has never used it.
	```{seealso}
	- [Native namespace packages](https://packaging.python.org/en/latest/guides/packaging-namespace-packages/#native-namespace-packages)
	- [PEP 420 – Implicit Namespace Packages](https://peps.python.org/pep-0420/)
	```

[stevepiercy]

@mauritsvanrees I committed all but this one suggestion, as I'd like your thoughts on it.

[davisagli]

This suggestion seems helpful to me.

[davisagli]

Aside from the pending suggestion, this looks good to me.