Specify all formats for specifying search paths in one place

[fricklerhandwerk]

Problem

Currently documentation on search paths is scattered over multiple locations:

• New CLI options (no anchors at time of writing): https://nixos.org/manual/nix/unstable/command-ref/new-cli/nix3-build.html
• Common options: https://nixos.org/manual/nix/unstable/command-ref/opt-common.html#opt-I
• Common environment variables: https://nixos.org/manual/nix/unstable/command-ref/env-common.html#env-NIX_PATH
• Configuration options: https://nixos.org/manual/nix/stable/command-ref/conf-file.html#conf-nix-path
• Release notes: https://nixos.org/manual/nix/stable/release-notes/rl-2.0.html?highlight=channel%3Anixos#release-20-2018-02-22

But none of them lists all possible ways to specify expression sources.

The best overview is provided by an external resource: https://nix.dev/reference/pinning-nixpkgs

Related: #7772

Proposal

Put all of that in one place, for example the documentation on NIX_PATH. Then remove the overview from nix.dev.

Checklist

• checked latest Nix manual (source)
• checked open documentation issues and pull requests for possible duplicates

Priorities

Add 👍 to issues you find important.

[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