use labs theme from external repo

[OriolAbril]

To reduce duplication and ensure a consistent look across the libraries,
I have moved the theme customization to https://github.com/pymc-labs/labs-sphinx-theme,
and extended the customization, especially for accent colors.

I am for now installing the theme from github to make sure everything continues to look the same,
but the theme already has a publish action (copied from the pymc theme) so it can be installed from pip instead.

Co-authored-by: @Mark0047

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📚 Documentation preview 📚: https://causalpy--385.org.readthedocs.build/en/385/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.60%. Comparing base (480816b) to head (9e4c885).
Report is 92 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #385   +/-   ##
=======================================
  Coverage   85.60%   85.60%           
=======================================
  Files          22       22           
  Lines        1716     1716           
=======================================
  Hits         1469     1469           
  Misses        247      247           

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[drbenvincent]

Looks good to me