enable fail_on_warning for readthedocs

[tkknight]

🚀 Pull Request

Description

Enables fail_on_warning when building on readthedocs.

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• benchmark_this Request that this pull request be benchmarked to check if it introduces performance shifts

[scitools-ci]

• Please see: SciTools/.github#253

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.18%. Comparing base (5e73fb2) to head (8b4887e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6909   +/-   ##
=======================================
  Coverage   90.18%   90.18%           
=======================================
  Files          91       91           
  Lines       24822    24822           
  Branches     4656     4656           
=======================================
  Hits        22385    22385           
  Misses       1669     1669           
  Partials      768      768           

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SciTools/.github#253 (comment)