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: plone/plone.app.dexterity#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.