Support $...$ math in headings

[fingolfin]

Fixes #127

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

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.85%. Comparing base (82d2039) to head (903db40).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/DocumentationTree.gi	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #358      +/-   ##
==========================================
- Coverage   92.85%   92.85%   -0.01%     
==========================================
  Files          13       13              
  Lines        2773     2784      +11     
==========================================
+ Hits         2575     2585      +10     
- Misses        198      199       +1     

Files with missing lines	Coverage Δ
gap/DocumentationTree.gi	96.08% <90.90%> (-0.15%)	⬇️

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