First of all, who is capable in moving the contributing section to markdown? I don’t think it is a reasonable expectation to improve it (significantly) when it’s still in xml. For once, the target audience (new contributors) will tend to run away as far as they possibly could from xml.
EDIT:
→ Convert contributing + using section from Docbook to CommonMark · Issue #120011 · NixOS/nixpkgs · GitHub
→ Nix Onboarding Story PRs / Issues