nixos/doc/configuration/profiles/qemu-guest.xml to CommonMark

[ryantm]

This tracks the conversion of nixos/doc/manual/configuration/profiles/qemu-guest.xml from DocBook to CommonMark per RFC 72.

Conversion instructions

1. Comment here that you intend to work on this. (Please also post if you stop working on this.)
2. Checkout a new branch of nixpkgs master.
3. Edit nixos/doc/manual/configuration/profiles.xml: change profiles/qemu-guest.xml to ../from_md/configuration/profiles/qemu-guest.section.xml.
4. Rename nixos/doc/manual/configuration/profiles/qemu-guest.xml to configuration/profiles/qemu-guest.section.md.
5. Convert the contents from Docbook to CommonMark. Follow conversion best practices. Use existing .md sections in the NixOS and Nixpkgs manual for inspiration.
6. Convert the CommonMark back to DocBook xml with:

   nixos/doc/manual/md-to-db.sh
   

7. Build from nixpkgs root dir with:

   nix-build nixos/release.nix -A manual.x86_64-linux
   

8. Start a preview server with:

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

9. Preview your work at http://0.0.0.0:8000/
10. Iterate on steps 5-9.
11. When it looks good, commit both the new .md file and the generated from_md .xml file, submit a PR and reference this issue.