Refactor documentation config and correct errors

[mdekstrand]

This cleans up the errors and warnings in the documentation build, and refactors conf.py to be more maintainable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.04%. Comparing base (febfec3) to head (c913acb).
Report is 562 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #428   +/-   ##
=======================================
  Coverage   91.03%   91.04%           
=======================================
  Files          47       47           
  Lines        3201     3204    +3     
=======================================
+ Hits         2914     2917    +3     
  Misses        287      287           

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