Trimming the set of bundled-by-default themes

[pradyunsg]

At some point we should also evaluate the stable of default themes and if that needs a refresh, but I think the basic work is more pressing.

All bundled themes are not maintained well. So it might be time to unbundle.

Originally posted by @tk0miya in https://github.com/sphinx-doc/sphinx/issues/10646#issuecomment-1186106560

I'm filing this as a separate issue, so that we don't lose track of this idea. :)