docs: Pin Sphinx version

[jmgate]

Until the theme updates accordingly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.67%. Comparing base (28c60f6) to head (30a5d86).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #254   +/-   ##
=======================================
  Coverage   78.67%   78.67%           
=======================================
  Files           6        6           
  Lines         830      830           
  Branches      143      143           
=======================================
  Hits          653      653           
  Misses        138      138           
  Partials       39       39           

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