Add warning for Python v3.14

[gitttt-1234]

This PR adds warnings to the documentation that Python 3.14 is not yet supported and updates install instructions to pin Python (e.g., --python 3.13) when using uv so users don’t accidentally end up on 3.14. It also updates the CI workflows to run on Python 3.13. In this PR, we also remove the deprecated flags --extra-index-url and --index-url for uv-based installations and update to --index flag in the installation docs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.54%. Comparing base (ff91433) to head (68bdc53).
⚠️ Report is 49 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #348      +/-   ##
==========================================
- Coverage   95.28%   93.54%   -1.74%     
==========================================
  Files          49       49              
  Lines        6765     7006     +241     
==========================================
+ Hits         6446     6554     +108     
- Misses        319      452     +133     

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