Configure the GitHub link in yardang's own docs

[timkpaine]

Description

fuma reads the repository link from html_theme_options, which renders the GitHub icon in the sidebar footer and derives the "Edit this page" link. yardang's own docs never set it, so after the switch to fuma the link disappeared. Add github_url to [tool.yardang] in pyproject.toml.

While wiring this up I found the theme docs point at a [tool.yardang.sphinx] namespace that yardang never reads — theme options are taken from [tool.yardang] directly, as the "Sphinx Options" section already documents. Corrected the four examples, and removed a sentence claiming searchlite_index_filename passes through the same nonextant namespace.

Note that github_url is fuma-specific, so yardang preview builds for the other themes log unsupported theme option 'github_url'. The build still succeeds; the option belongs to fuma and the main site build uses it.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup
• CI / build configuration
• Other (describe below)

Checklist

• Linting passes (make lint)
• Tests pass (make test)
• New tests added for new functionality
• Documentation updated (if applicable)
• Changelog / version bump (if applicable)

No test covers this: it is repo configuration plus docs. Verified by generating the config (html_theme_options = {'github_url': 'https://github.com/python-project-templates/yardang'}) and by building a sample project, which emits the link and aria-label="GitHub" in the rendered HTML. Suite: 118 of 119 pass; test_all.py::test_cli fails locally because the optional sphinxcontrib_rust extra is not installed, which is unrelated.

[github-actions]

Test Results

119 tests  ±0   119 ✅ ±0   21s ⏱️ -3s
  1 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit a5a25d7. ± Comparison against base commit 91ddc0f.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.02%. Comparing base (f942c03) to head (a5a25d7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #166   +/-   ##
=======================================
  Coverage   89.02%   89.02%           
=======================================
  Files          16       16           
  Lines        2096     2096           
  Branches      214      214           
=======================================
  Hits         1866     1866           
  Misses        160      160           
  Partials       70       70           

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