Split contribution docs into an isolated subsection and unify all Nix contribution docs into one site

Observations

Currently nixpkgs, nix and the docs team have separate contribution information areas. Currently they use CONTRIBUTING files which can get long and don't necessarily have nice space for presenting justification or vision, which leads to contributor burnout due to lack of value alignment leading to bike shedding.

Problem

There's not a lightweight to edit place for contribution docs that's centralized, has nice navigation, and working search.

Approaches

infinisil noted that it would be preferable to keep docs inside nixpkgs. I have no horse in this race; if we link it well it's probably fine regardless of where the files go.

The central idea of this is creating a place like the rustc developer book which is easy to edit (not quite a wiki but more chill than official docs) and hopefully stays up to date; it provides explanations of how internals work, high level architecture explanations, and reasoning for things. https://rustc-dev-guide.rust-lang.org/

https://logs.nixos.dev/room/!avYyleMexqjFHoqrME:nixos.org/?anchor=$1a-p0jIFGfppHqFadSnQIpoX40mUALH9qt7griX9qWk&offset=-10

Willing to help?

Maybe, feels like this might itself die to the bike shed and I'm afraid to touch it.

Priorities

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Review the existing CONTRIBUTING files across nixpkgs, nix, and the docs team, then compare their organization with the linked Rust compiler development guide. A complete effort would establish an agreed location and structure for unified contribution documentation with navigation and search, but the issue does not identify specific files or a settled implementation path.