Fix mixed explicit and implicit chapter contexts

[fingolfin]

Create missing chapter and section nodes lazily when grouped items
are inserted so mixed explicit and auto-generated layouts do not
crash.

Fixes #279.

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

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.82%. Comparing base (3d0e6cf) to head (d4ca386).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/DocumentationTree.gi	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #310      +/-   ##
==========================================
+ Coverage   84.69%   84.82%   +0.12%     
==========================================
  Files          13       13              
  Lines        2281     2280       -1     
==========================================
+ Hits         1932     1934       +2     
+ Misses        349      346       -3     

Files with missing lines	Coverage Δ
gap/DocumentationTree.gi	94.04% <66.66%> (+0.23%)	⬆️

... and 1 file with indirect coverage changes

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