660806066a
This allows one to specify "related packages" in NixOS that get rendered into the configuration.nix(5) man page. The interface philosophy is pretty much stolen from TeX bibliography. See the next several commits for examples.
To build the manual, you need Nix installed on your system (no need
for NixOS). To install Nix, follow the instructions at
https://nixos.org/nix/download.html
When you have Nix on your system, in the root directory of the project
(i.e., `nixpkgs`), run:
nix-build nixos/release.nix -A manual.x86_64-linux
When this command successfully finishes, it will tell you where the
manual got generated.