feat: add sphinx-sitemap dependency and configuration for sitemap generation

[imtherealnaska]

Which issue does this PR close?

• Part of Improve search engine / agent discoverability for DataFusion website #23258

What changes are included in this PR?

There are 2 PR's for this issue. This one contains the changes for sitemap.xml

• added the sphinx-sitemap build dependency.
• enabled the sphinx_sitemap extension, set html_baseurl, and set sitemap_url_scheme = "{link}"

Are these changes tested?

Yes. Tested by building the docs and verifying the files getting generated in the build/

Are there any user-facing changes?

No.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.14%. Comparing base (6b051b0) to head (d2089db).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24332      +/-   ##
==========================================
- Coverage   81.14%   81.14%   -0.01%     
==========================================
  Files        1112     1112              
  Lines      386933   386933              
  Branches   386933   386933              
==========================================
- Hits       313976   313958      -18     
- Misses      54467    54480      +13     
- Partials    18490    18495       +5     

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[Jefffrey]

should be good once merge conflicts are resolved