Add documentation for how to administer Read the Docs for documentation

[stevepiercy]

Closes https://github.com/plone/plone-sphinx-theme/issues/10

Closes #1604

Closes #1600

Depends on merge of https://github.com/plone/volto/pull/6030 because its custom.css file pollutes default styles, turning the plone.api docs into an unreadable mess, as shown in https://plone6--1667.org.readthedocs.build/plone.api/api/user.html.

Also depends on the replacement of our custom search with Nuclia search.

• https://github.com/plone/documentation/pull/1529
• https://github.com/plone/training/pull/792