Refactor item type metadata

[fingolfin]

Centralize manual item type metadata, add support for Constr, Fam,
InfoClass, Meth, and the Cat, Coll, and Repr aliases, and fix
filter default chapter buckets.

Change InstallMethod and InstallOtherMethod to document entries as
Meth by default, update XML emission to use metadata-driven GAPDoc
tags, and refresh docs and expected outputs.

AI disclosure: Codex assisted with implementation, documentation
updates, and test fixture regeneration.

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

[codecov]

Codecov Report

❌ Patch coverage is 99.19355% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.00%. Comparing base (22a82a6) to head (d1b89d3).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/Parser.gi	99.07%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #350      +/-   ##
==========================================
+ Coverage   91.58%   92.00%   +0.41%     
==========================================
  Files          13       13              
  Lines        2638     2688      +50     
==========================================
+ Hits         2416     2473      +57     
+ Misses        222      215       -7     

Files with missing lines	Coverage Δ
gap/AutoDocMainFunction.gi	87.61% <100.00%> (+0.15%)	⬆️
gap/Parser.gd	100.00% <100.00%> (ø)
gap/ToolFunctions.gi	92.57% <100.00%> (+0.23%)	⬆️
gap/Parser.gi	96.56% <99.07%> (+0.84%)	⬆️

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