[DOCS] use pip and apt to install packages for RTD builds

[zacharyburnett]

Tasks

• update or add relevant tests
• update relevant docstrings and / or docs/ page
• Does this PR change any API used downstream? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see changelog readme for instructions)
  • run regression tests with this branch installed (stpipe@git+https://github.com/<fork>/stpipe.git@<branch>)
    • jwst regression test
    • romancal regression test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.66%. Comparing base (a75274a) to head (82d5c0b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #311   +/-   ##
=======================================
  Coverage   95.66%   95.66%           
=======================================
  Files          35       35           
  Lines        3391     3391           
=======================================
  Hits         3244     3244           
  Misses        147      147           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[braingram]

Thanks!