Convert Gnome from Docbook to CommonMark

[ryantm]

This issue tracks the conversion of /doc/languages-frameworks/gnome.xml from Docbook to CommonMark per RFC 72.

If you intend to work on this conversion, please comment below!

Instructions for performing this conversion

1. Post in this issue that you intend to work on it. (Please also post if you stop working on it)
2. Checkout a new branch of nixpkgs master.
3. Edit doc/languages-frameworks/index.xml: change gnome.xml to gnome.section.xml.
4. Convert the contents from Docbook to CommonMark. Follow conversion best practices. Use existing .md sections for inspiration.
5. Convert the contents from Docbook to CommonMark. Use existing .md sections for inspiration.
6. Use nix-build from the doc/ folder to build the manual.
7. Preview the manual on your computer at http://0.0.0.0:8000/manual.html after running from doc/ folder:

nix-shell -p python3 --run 'python3 -m http.server --directory ./result/share/doc/nixpkgs'

8. When it looks good, submit a PR and reference this issue.

[jtojnar]

I can try to tackle this, hopefully we will be able to figure out how to make it functionally equivalent, even if semantic equivalence is out.

[jtojnar]

I want to finish #101537 first to avoid conflicts, though.

[Mic92]

This was merged now.