Several "common" warnings for `gapmacro.tex` based old documentation

[fingolfin]

I am currently revising the doc/make_doc scripts of a bunch of packages and am running in a couple issues in multiple packages each:

• Warning: Chapter has no sections -- I think this would be safe to ignore. See. e.g.
  • Revise make_doc gap-packages/CaratInterface#40
  • Revise make_doc gap-packages/cryst#57
  • Revise make_doc gap-packages/format#9
  • Revise make_doc gap-packages/fwtree#10
  • Revise make_doc gap-packages/grpconst#12
  • Revise make_doc gap-packages/liepring#32
  • Revise make_doc gap-packages/nilmat#8
  • Revise make_doc gap-packages/radiroot#13
  • Revise make_doc gap-packages/rds#5
  • Revise make_doc gap-packages/rig#21
  • Revise make_doc gap-packages/sglppow#10
  • Revise make_doc gap-packages/xgap#36
• Warning: did not find a .bbl file ... ok? -- I don't understand what this is about. See e.g.
  • Revise make_doc gap-packages/cohomolo#39 and then e.g. the log https://github.com/gap-packages/cohomolo/actions/runs/17701982036/job/50308832917
  • Revise make_doc gap-packages/rig#21
• ...

I may add more warnings as I encounter them.

Anyway: it would be good to be able to ignore these in this action without having to disable all warnings checks

[fingolfin]

I guess alternatively we could patch gapmacro.tex to just not issue this pointless "Chapter has no sections" warning

[fingolfin]

Turns out it is etc/convert.pl which issues this warning.

See gap-system/gap#6115

[fingolfin]

The did not find a .bbl file warnings also come from etc/convert.pl and are caused by the lack of a bibliography: turns out that script (which produces a HTML version of the manual) always produces a bibliography page, unconditionally. Arguably it should have a way to disable that if there is no bib...