docs: convert manual chapters to .autodoc

[fingolfin]

Rewrite the hand-written manual chapters in .autodoc and update
the dogfood fixtures to validate the generated manual output.

Refresh manual cross-references and clean stale generated chapter
XML files before regenerating expected output.

Resolves #176

Co-authored-by: Codex codex@openai.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.85%. Comparing base (94ab263) to head (cebb898).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #314      +/-   ##
==========================================
+ Coverage   85.72%   85.85%   +0.12%     
==========================================
  Files          13       13              
  Lines        2438     2439       +1     
==========================================
+ Hits         2090     2094       +4     
+ Misses        348      345       -3     

Files with missing lines	Coverage Δ
gap/AutoDocMainFunction.gi	80.76% <100.00%> (ø)
gap/Magic.gd	100.00% <ø> (ø)
makedoc.g	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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