Add GUI README fixes + more informative docs debug msg

[luarss]

Addresses feedback in #9683

@oharboe @maliberty FYI

[gemini-code-assist]

Code Review

This pull request improves the debug message for the documentation generation script, making it easier to find formatting errors in README files. It also adds new documentation for a GUI feature.

My review includes a fix for a bug in the new debug logic that could fail with duplicate headers, and a minor formatting suggestion for the new documentation to ensure consistent heading levels.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[luarss]

Also fixed a bug with image render on readthedocs.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[oharboe]

I'm skeptical of complexity and feature rich CI.

My main priority would be to have simple fast local tests that require no maintainence in .yaml scripts or Jenkins.

#9733

@hzeller FYI

[oharboe]

I am skeptical of having an edit, test, debug cycle that involves CI. This flow should be entirely local and fast.

Ideally, CI is just a go-no go catchall if something wasn't caught locally and it should provide an easy to spot command to reproduce the problem locally for fast local edit, test, debug cycles.

[luarss]

@oharboe This is complementary to your PR. It just adds more informative messages to the same base md_roff_compat.py script for debugging. I am also in favor of the local dev flow