RTD: Coordinated docs as subprojects to core doc

RTD subproject idea was floated at the Astropy Coordination Meeting 2025 as a possible way to tied Coordinated docs to core lib doc. Since then, @Cadair had linked the coordinated docs to https://docs.astropy.org/en/latest/ under Documentation top drop-down menu. Therefore, the issue of where those docs should be hosted has moved beyond https://github.com/astropy/astropy/issues/18367 into this issue instead.

Also see:

* https://github.com/astropy/astropy-project/issues/522
* https://github.com/astropy/astropy/issues/8221
* https://github.com/astropy/astropy.github.com/issues/141

Start by reviewing the current coordinated-docs link under the Documentation menu and the related issues #18367, #522, #8221, and #141. The issue does not name files or tests; done would require an agreed, documented decision about where coordinated documentation is hosted and how it relates to the core documentation.

Written by the indexing model from the issue text.