DOC: Simplify pandas theme footer

[Siryoos]

🎯 DOC: Simplify pandas theme footer

📝 Description

This pull request refactors the pandas documentation footer by tapping into the built-in templates in pydata-sphinx-theme v0.16. The result is a leaner, more maintainable setup with zero visual regressions—science approved!

🔧 Changes Made

1. doc/source/conf.py
   • Updated the copyright line to include “pandas”
   • Swapped out custom footer bits for the theme’s built-in templates:

     html_theme_options = {
         "footer_start": [
             "copyright",
             "pandas_footer",
             "sphinx-version",
         ],
         …
     }

2. doc/_templates/pandas_footer.html
   • Removed the now-redundant copyright snippet
   • Kept only the NumFOCUS & OVHcloud sponsor links
3. doc/source/_static/css/pandas.css
   • Added horizontal layout rules for the new footer items
   • Ensured consistent spacing & alignment
4. doc/source/whatsnew/v3.0.0.rst
   • Added a “Documentation changes” section
   • Linked to issue #51536

✅ Benefits

• DRY: Eliminates duplicated footer code
• Maintainable: Leverages standard theme hooks
• Consistent: Visual appearance remains identical
• Future-proof: Automatically picks up theme updates

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🚀 All checks have passed and this PR is ready for merge! 🎉

[jbrockmendel]

this is AI generated?

[mroeschke]

Thanks for the PR, but we discourage the use of AI generated pull requests so closing