feat(themes): add bundled theme defaults and yardang preview

[timkpaine]

Ship per-theme defaults and a preview workflow so the docs can be compared across multiple Sphinx themes.

• pyproject: add themes extra (shibuya, sphinxawesome-theme)
• build.py: add BUNDLED_THEMES and _resolve_custom_asset for per-theme CSS/JS defaults; add html_output_dir so the HTML output dir can be redirected
• cli.py: add yardang preview to render the site once per theme; build gains an --output option
• bundled shibuya.css / sphinxawesome_theme.css defaults
• docs: document the themes extra, custom-css/js precedence, and previewing
• ci: build with the working copy, render per-theme previews, publish to gh-pages
• tests: cover bundled themes and preview behaviour

[codecov]

Codecov Report

❌ Patch coverage is 91.24088% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.66%. Comparing base (c894454) to head (03af5aa).

Files with missing lines	Patch %	Lines
yardang/cli.py	62.96%	7 Missing and 3 partials ⚠️
yardang/build.py	93.93%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #116      +/-   ##
==========================================
+ Coverage   80.98%   82.66%   +1.68%     
==========================================
  Files           8        9       +1     
  Lines        1262     1373     +111     
  Branches      138      145       +7     
==========================================
+ Hits         1022     1135     +113     
+ Misses        201      198       -3     
- Partials       39       40       +1     

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