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: https://github.com/spacetelescope/stcal/pull/267 or https://github.com/spacetelescope/stcal/pull/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
• [x] updated relevant documentation
• [ ] updated relevant milestone(s)
• [x] added relevant label(s)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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