Update docs to include polygon tutorial

[ajfriend]

Updating the docs ahead of releasing 4.0:

• Making sure they still build without issues
• Adding tutorial for the new polygon interface (create docs from running a jupyter notebook)
• Closes Docs build action should trigger on PRs #356
• Closes Update codecov action #350

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (89c3720) to head (1cd54c6).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #355   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          319       319           
=========================================
  Hits           319       319           

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[ajfriend]

Closes #350

[ajfriend]

It looks like pypa/[email protected] is working fine, but let's remove that here and do it in a separate PR: #358

[ajfriend]

Ready to land, pending reviews