Updating the documentation sphinx environment

[ceb8]

This pull request updates the documentation sphinx environment to use sphinx_astropy.conf.v2 and the pydata-sphinx-theme. It also switches to using the astroquery-specific logo, and reorganizes the documentation main page.

This does not change the substance of the documentation at all (except in a few small places where the wording had to be changed due to the new organization).

There are two outstanding issues that I see:

• Do we want to use this logo? I know we have talked about needing a new logo, but that has not happened so I went with the one we had. I can always revert to the standard Astropy logo.
• I had to comment out the MOCK_MODULES lines in conf.py because they break the documentation build. I don't understand their purpose, so I didn't know how to fix that.

[bsipocz]

@ceb8 - please do open discussion before jumping into this. E.g. I have already started working on the switch.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.37%. Comparing base (af2d6f4) to head (32ee68d).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3281   +/-   ##
=======================================
  Coverage   69.37%   69.37%           
=======================================
  Files         232      232           
  Lines       19686    19686           
=======================================
  Hits        13657    13657           
  Misses       6029     6029           

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[bsipocz]

And I said it before, I quite dislike this particular logo. We don't have better but that doesn't mean we need to start using this one (which we also don't have the proper attributes any more as it was created as a proof of concept during a hackday).