Documentation: Merge NixOS Wiki article on NixOS modules into the manual

[fricklerhandwerk]

Problem

Currently there are two prominent pieces of documentation on NixOS modules. This is confusing for beginners in two ways:

1. It misleads them into thinking the NixOS Wiki is the authoritative and curated source of documentation, which it is not.
2. It requires them switching back and forth between different resources, which have high overlap but are both incomplete ("tab explosion").

Related:

• https://discourse.nixos.org/t/usability-study-session-4/21422
• https://discourse.nixos.org/t/usability-study-session-10/21456
• https://discourse.nixos.org/t/usability-study-session-5/21424

Proposal

Merge into NixOS Wiki: NixOS Modules into NixOS manual: Writing NixOS modules and link from the Wiki article to the manual.

This can be done in multiple steps, but in any case will be quite some work untangling the overlap and the differences.

Checklist

• [x] checked latest Nixpkgs manual (source) and latest NixOS manual (source)
• [x] checked open documentation issues for possible duplicates
• [ ] checked open documentation pull requests for possible solutions

Note: Did not check the entire list of possible PRs.

[nixos-discourse]

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

https://discourse.nixos.org/t/tweag-nix-dev-update-44/25546/1