Migrate plone.app.dexterity docs into Plone 6 Documentation

[MrTango]

Description

To make it easier it would be nice to have the external docs from https://github.com/plone/plone.app.dexterity also as a myst version. The final docs will be part of the new mains docs, no sub repo.

[stevepiercy]

Refs: https://github.com/plone/plone.app.dexterity/issues/360

[stevepiercy]

The source files now reside at plone.app.dexterity, and are awaiting a content expert to review them and place them in their home.

[MrTango]

I'm working on the chapter "Rich text, markup and transformations":

Python docs including RichTextValue details should go into fields. I'm still oundering if the template part should go into templates or stay with the RichText field? @petschki @jensens @pbauer opinions on this?

[MrTango]

see https://github.com/plone/documentation/pull/1648

[jensens]

I'm still oundering if the template part should go into templates or stay with the RichText field?

IMO it is better keeps things together in one place here. In "templates" point readers to this place. I don't like it if information is scattered around the docs.