Add documentation contribution guide to CONTRIBUTING.md

[aman-coder03]

Summary

Fixes #90

Adds a ### Documentation section to CONTRIBUTING.md covering how to build the docs locally and how to add example notebooks, including how to test them with nbmake and guidelines for what makes a good notebook contribution.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.96%. Comparing base (5b8382d) to head (9b54345).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
+ Coverage   91.36%   92.96%   +1.60%     
==========================================
  Files           6        6              
  Lines         799      825      +26     
==========================================
+ Hits          730      767      +37     
+ Misses         69       58      -11     

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[martinfleis]

The exact shape of contributing guides across PySAL will be discussed and synchronised. Putting this on hold.

[aman-coder03]

Understood, happy to wait. Let me know once the discussion is settled and i will update accordingly