Move `plone/plone.app.dexterity` docs into `plone/documentation`

[stevepiercy]

During the Plone 6 Documentation Sprint, we proposed to move the docs for plone/plone.app.dexterity into plone/documentation, instead of pulling them in as a submodule. See The Dexterity-reference from the training and the dexterity docs should both move into docs · Issue #1298 · plone/documentation · GitHub for details.