[fingolfin]

Plain text read while no chapter is active was dropped silently. Text
added to a chapter after one of its sections, or to a section after
one of its subsections, is placed after that child, so GAPDoc renders
it as part of the last section or subsection. Both now print a
warning with the source position.

The commands @chapter, @appendix, @section and @subsection move to a
new section "Structuring the documentation" at the start of the
comments chapter, which also explains where plain text ends up.

Fixes #383

Assisted-by: Generative AI

[codecov]

Codecov Report

❌ Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.11%. Comparing base (fe784fd) to head (9c78b9e).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/Parser.gi	97.14%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #390      +/-   ##
==========================================
+ Coverage   93.00%   93.11%   +0.10%     
==========================================
  Files          15       15              
  Lines        3144     3179      +35     
==========================================
+ Hits         2924     2960      +36     
+ Misses        220      219       -1     

Files with missing lines	Coverage Δ
gap/Parser.gi	96.90% <97.14%> (+0.19%)	⬆️

... and 1 file with indirect coverage changes

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