Add a workflow to automatically rebuild RTD when the theme is released

Because the `astropy-unified` theme now has shared configuration over all coordinated packages (specifically the top navigation bar), there's a pretty strong motivation to keep all the various RTD projects in sync with the latest theme release.

SunPy solves this by having a script which triggers rebuilds for all active versions of all RTD subprojects: https://github.com/sunpy/sunpy-sphinx-theme/blob/main/tools/rebuild_all_rtd.py

To directly adopt this here we would either need to use subprojects or we would need one RTD account with access to all the projects to be able to make a single RTD token for the script. xref https://github.com/astropy/astropy/issues/18367

Start by reading SunPy's tools/rebuild_all_rtd.py and the linked astropy/astropy#18367 discussion. Determine whether subprojects or a shared Read the Docs token can provide access to all projects. Done means an automated workflow triggers rebuilds for all active versions when the theme is released.

Written by the indexing model from the issue text.