Integrate sphinx-fuma and sphinx-searchlite

[timkpaine]

Description

Add sphinx-fuma as a first-class preview theme and enable sphinx-searchlite by default across themes. Use the published packages instead of vendored source, add configuration documentation, and cover theme/search configuration.

Companion searchlite changes: python-project-templates/sphinx-searchlite#6

Local test suite passes 90 of 91 tests. The remaining Rust API rendering failure reproduces on main with the same environment (docutils rejects sphinx_rust’s _StrPath).

Type of Change

• New feature
• Documentation update
• Refactor / code cleanup

Checklist

• Linting passes (make lint)
• Tests pass (make test) — 90/91; remaining failure reproduces on main
• New tests added for new functionality
• Documentation updated
• Changelog / version bump (not requested)

[codecov]

Codecov Report

❌ Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.53%. Comparing base (18157b4) to head (bb052e6).

Files with missing lines	Patch %	Lines
yardang/build.py	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
+ Coverage   85.38%   85.53%   +0.15%     
==========================================
  Files          15       15              
  Lines        1909     1936      +27     
  Branches      203      204       +1     
==========================================
+ Hits         1630     1656      +26     
  Misses        216      216              
- Partials       63       64       +1     

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