GH actions - Exclude CI and main for 'docs' changes

[jomey]

Do not run the CI or main actions once there are only changes to documentation. On the other hand, trigger the docs only when there are changes to that.

Reference:

• https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#example-excluding-paths
• https://docs.readthedocs.io/en/stable/build-customization.html#cancel-build-based-on-a-condition

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.19%. Comparing base (76d8c92) to head (673af60).
Report is 13 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #139   +/-   ##
=======================================
  Coverage   70.19%   70.19%           
=======================================
  Files          12       12           
  Lines         416      416           
=======================================
  Hits          292      292           
  Misses        124      124           

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