Add source-file path and raw-source link to page footer

[tony]

Summary

• Add a source-file provenance line to every documentation page footer, showing the repo-relative path (e.g. docs/configuration.md) and a "Raw source" link pointing to the raw file on GitHub
• Guard the new section on theme_source_repository and page_source_suffix so it only appears on content pages — generated pages like genindex and search are excluded automatically

Design decisions

• Reuse existing theme options: The footer constructs its URL from source_repository, source_branch, and source_directory — all already declared in theme.conf and wired through merge_sphinx_config(). No new configuration surface needed.
• page_source_suffix as the guard: Sphinx leaves this variable falsy for generated pages, providing a natural exclusion mechanism without maintaining a hardcoded page-name list.
• /raw/ URL path: GitHub's /raw/{branch}/{path} endpoint serves the unprocessed source file, matching the "machine-readable" intent. GitLab supports a similar endpoint at the same path segment.

Verification

Verify the source line appears on content pages:

$ grep -c "page-source" docs/_build/html/index.html

Verify the source line does not appear on generated pages:

$ grep -c "page-source" docs/_build/html/genindex.html

Test plan

• uv run pytest — all existing tests pass
• Built docs show Source: docs/index.md · Machine-readable: Raw source in footer on content pages
• genindex.html and search.html have no source line
• Raw source link resolves to correct GitHub URL (/raw/main/docs/...)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.82%. Comparing base (3b18b5d) to head (64c4932).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #46   +/-   ##
=======================================
  Coverage   91.82%   91.82%           
=======================================
  Files         220      220           
  Lines       17776    17776           
=======================================
  Hits        16322    16322           
  Misses       1454     1454           

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