Use fork of mdformat-gfm for Markdown formatting

[alexreinking]

Rather than exclude a file from formatting, I chose to fix the formatter. It was very easy to do.

I tried to open an issue with upstream, but the maintainer decided the bug was with GitHub's inconsistent rendering, rather than with the formatting logic. See hukkin/mdformat-gfm#58

See also #9054

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)