feat: DocBook -> CommonMark

[ryantm]

Output is CommonMark with some syntax extensions:

• DD lists
• heading anchors

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/happy-docsgiving/10214/28

[zimbatm]

@ryantm this PR looks ready isn't it?

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-03-14-documentation-team-meeting-notes-32/26351/1

[pennae]

we would like to propose that this PR be updated to support examples as in NixOS/nixpkgs#220116, which is very close to how pandoc represented examples (but can actually reconstruct a docbook title tag). that's the update discussed in the docs team meeting notes.

[asymmetric]

@infinisil you're looking into updating this right?

[ryantm]

@ryantm this PR looks ready isn't it?

I've been testing the output of it in my version of the Nixpkgs manual for over a year now. https://ryantm.github.io/nixpkgs/functions/library/asserts/#sec-functions-library-asserts

So, yes, probably.

[asymmetric]

I propose we merge this and address any further needs in follow-up PRs, since a) @ryantm has tested it for a long time and it works; and b) he's looking for new maintainers, so this could be a nice handover line.