switch docs theme to sphinx_rtd, unpin sphinx

[braingram]

This PR switches the sphinx theme to sphinx_rtd_theme (as was done for jwst, stpipe, romancal, etc).

This also fixes the broken search.

If this conflicts with:
#267
or
#270

I suggest we wait until after those are merged.

Link to built docs: https://stcal--272.org.readthedocs.build/en/272/index.html

Checklist

• added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.38%. Comparing base (cd47dc6) to head (bd72ed5).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #272   +/-   ##
=======================================
  Coverage   84.38%   84.38%           
=======================================
  Files          41       41           
  Lines        7557     7557           
=======================================
  Hits         6377     6377           
  Misses       1180     1180           

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