Preserve relative paths in copied root pages

[timkpaine]

yardang copies the configured documentation root to repository-level index.md, which changes the base directory used by relative links and assets.

• Rebase relative Markdown links, images, reference definitions, and active raw HTML href, src, and srcset values.
• Preserve external URLs, query strings, fragments, code examples, and HTML comments.
• Copy locally referenced raw HTML assets into matching paths under the Sphinx output directory.
• Cover mixed data/local srcset values, angle-delimited paths, unquoted attributes, and fenced content.

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.30%. Comparing base (b74a1df) to head (290910b).

Files with missing lines	Patch %	Lines
yardang/markdown.py	91.71%	5 Missing and 9 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #124      +/-   ##
==========================================
+ Coverage   83.72%   85.30%   +1.58%     
==========================================
  Files          10       12       +2     
  Lines        1456     1708     +252     
  Branches      150      184      +34     
==========================================
+ Hits         1219     1457     +238     
- Misses        198      203       +5     
- Partials       39       48       +9     

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