Add @Appendix command similar to @Chapter

[fingolfin]

Fixes #76

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.23%. Comparing base (3881568) to head (0191085).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #353      +/-   ##
==========================================
+ Coverage   92.00%   92.23%   +0.23%     
==========================================
  Files          13       13              
  Lines        2688     2730      +42     
==========================================
+ Hits         2473     2518      +45     
+ Misses        215      212       -3     

Files with missing lines	Coverage Δ
gap/AutoDocMainFunction.gi	88.71% <100.00%> (+1.09%)	⬆️
gap/DocumentationTree.gd	100.00% <100.00%> (ø)
gap/DocumentationTree.gi	96.23% <100.00%> (+0.25%)	⬆️
gap/Magic.gd	100.00% <ø> (ø)
gap/Magic.gi	74.12% <100.00%> (+0.26%)	⬆️
gap/Parser.gi	96.59% <100.00%> (+0.02%)	⬆️

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