switch to sphinx_rtd_theme

[braingram]

Similar to other stsci repos this PR switches the sphinx theme to sphinx_rtd_theme (from stsci_rtd_theme).

See https://stpipe--159.org.readthedocs.build/en/159/api/stpipe.Step.html#stpipe.Step for an example of an improvement with the new theme. The parameters render with a separation between the names and types unlike on main:
https://stpipe.readthedocs.io/en/latest/api/stpipe.Step.html#stpipe.Step

The oldestdeps failure is unrelated, see #158

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.46%. Comparing base (d056adc) to head (46137f0).
Report is 72 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #159   +/-   ##
=======================================
  Coverage   69.46%   69.46%           
=======================================
  Files          24       24           
  Lines        1634     1634           
=======================================
  Hits         1135     1135           
  Misses        499      499           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.