pin `sphinx_rtd_theme` to newer version

[zacharyburnett]

https://jira.stsci.edu/browse/SCSB-80

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

Patch and project coverage have no change.

Comparison is base (b3a98f7) 74.41% compared to head (b23ef40) 74.41%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #184 +/- ## ======================================= Coverage 74.41% 74.41% ======================================= Files 29 29 Lines 5944 5944 ======================================= Hits 4423 4423 Misses 1521 1521 ```

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