Clarify cz_customize deprecation warning with rationale link

[ttw225]

Description

https://commitizen-tools.github.io/commitizen/customization/config_file/

• The existing warning said that cz_customize is likely to be removed or renamed in the next major release, but did not point to any background discussion or roadmap.
• This change makes it explicit that the change is still under discussion, that users can continue using cz_customize for now, and that Deprecate the cz_customize convention and allow for overriding functionality #1385 is the place to track the rationale and design/alternative discussions.

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: [Cursor] following the 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)

Expected Behavior

• Clarifies the cz_customize warning in the configuration docs.
• Adds a reference to Deprecate the cz_customize convention and allow for overriding functionality #1385 so users can follow the rationale, options, and current status.

Steps to Test This Pull Request

Additional Context

[bearomorphism]

Thanks!