Support Markdown heading aliases

[fingolfin]

Allow #, ##, ### headings in .autodoc and #! comments as aliases
for @Chapter/@Section/@subsection. Add parser coverage in
misc tests and update autoplain worksheet expected output.

Resolves #213

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

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.66%. Comparing base (88c828a) to head (ffc985f).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/Parser.gi	93.75%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #308      +/-   ##
==========================================
+ Coverage   84.53%   84.66%   +0.12%     
==========================================
  Files          13       13              
  Lines        2244     2276      +32     
==========================================
+ Hits         1897     1927      +30     
- Misses        347      349       +2     

Files with missing lines	Coverage Δ
gap/Parser.gi	81.38% <93.75%> (+0.54%)	⬆️

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