Add ability to add extra cross-project search projects on an opt-in basis

[Cadair]

Describe the feature

For SunPy it would be a nice addition to opt-in to also searching the astropy documentation.

Proposed solution

I'm thinking that it would be good to have something like the default RTD option for subprojects but for "include ":

I think it would be nice to have this configurable where you can configure one or more groups which the user can then enable, i.e.:

rtd_search_extras = {
    "astropy": {"default_enabled": False, "projects": [{"slug": "astropy", "link": "https://docs.astropy.org"}]}
}