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.

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📚 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