Split contribution docs into an isolated subsection and unify all Nix contribution docs into one site about nix.dev

[lf-]

NixOS/nixpkgs#270696 (comment)

this is an example of a contribution that the contributor didn't know the right place to put it and would have benefited from a high level view of where stuff goes. i believe we have one but it's not in the path where we look for things as contributors.

[fricklerhandwerk]

I think these are two issues:

1. Sort the various documentation types into their places (editorial)
2. Make a unified presentation (technical)

It seems the @NixOS/documentation-team has mostly figured out where to put what generally:

• Reference documentation (including examples): in the source as far as possible, rendered in reference manuals
• Contribution guides: in-tree, close to the source
• Design documentation: in the source
• User guides: nix.dev
• Tutorials: nix.dev

There are still not enough places where the principle and rationale is written down or linked to help contributors figure it out by themselves. It's made worse that all of that is still implemented only to a small degree, making it almost impossible to infer from context whether something fits in. Also we sometimes seem to be struggling with determining what's what for a given PR, which probably cannot be solved by writing more things down because it very much depends on the situation.