Update sphinx configuration to use new astropy-unified theme

[Cadair]

This PR migrates this repo to use the new astropy-unified sphinx theme.

For more information about this project see astropy/astropy-project#522

This is an automated update made by the batchpr tool 🤖.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.56%. Comparing base (e2a808f) to head (2659867).
⚠️ Report is 22 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #922   +/-   ##
=======================================
  Coverage   96.56%   96.56%           
=======================================
  Files           8        8           
  Lines        1571     1571           
=======================================
  Hits         1517     1517           
  Misses         54       54           

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[mwcraig]

Merging, since the fail is unrelated....

[Cadair]

@mwcraig wins the christmas turkey ❤️

[mwcraig]

🦃🎄😁