Replace sphinx-rust with sphinxcontrib-rust

[timkpaine]

sphinx-rust has not been released since April 2024 and pins sphinx~=7.3, which held the whole dependency set on Sphinx 7 and made current versions of pydata-sphinx-theme and sphinx-book-theme unresolvable.

sphinxcontrib-rust is actively maintained and supports Sphinx 7 through 9. It is GPL-3.0 rather than MIT, so it stays confined to the opt-in rust extra, and it builds a sphinx-rustdocgen binary on install, which requires a Rust toolchain.

The configuration is not a drop-in replacement, so [tool.yardang.sphinx-rust] changes shape: crates becomes a name to directory mapping, doc-dir replaces the implicit output location, and doc-formats and viewcode give way to rustdoc-fmt, visibility, strip-src and generate-mode.

Pages are now generated rather than written by hand, so the Rust section of the API example links to the generated page instead of declaring directives itself.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.19%. Comparing base (0f5dc60) to head (88d647d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #151      +/-   ##
==========================================
+ Coverage   85.48%   86.19%   +0.70%     
==========================================
  Files          15       16       +1     
  Lines        1936     2021      +85     
  Branches      204      209       +5     
==========================================
+ Hits         1655     1742      +87     
- Misses        216      217       +1     
+ Partials       65       62       -3     

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