docs: add warning about future removal of cz_customize

[Stoiicc]

Closes #1710

Description

Added a warning in the documentation to inform users that cz_customize is likely to be removed or renamed in a future major release.

Checklist

• I have read the contributing guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run uv run poe all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

Documentation Changes

• Run uv run poe doc locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external) in the documentation

Expected Behavior

Readers of the documentation are clearly informed that cz_customize may be removed or renamed in the next major release.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.52%. Comparing base (8832804) to head (2b111a7).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1753      +/-   ##
==========================================
+ Coverage   97.92%   98.52%   +0.60%     
==========================================
  Files          60       60              
  Lines        2604     2582      -22     
==========================================
- Hits         2550     2544       -6     
+ Misses         54       38      -16     

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[bearomorphism]

Thanks!