Add generated options documentation as another doc source

[elkowar]

The data that the generated JSON documentation from options can provide should be used as a documentation source. This would imply:

• adding an abstraction for different doc sources, some of which might not need any hashing or stuff like that
• generating the options documentation for NixOS options as well as home-manager / finding where they are by default
• Adding the documentation source.

[elkowar]

with import <nixpkgs> {}; 
let 
  eval = import (pkgs.path + "/nixos/lib/eval-config.nix") { modules = []; }; 
  opts = (nixosOptionsDoc { options = eval.options; }).optionsJSON; 
in 
  runCommandLocal "options.json" { inherit opts; } "cp $opts/share/doc/nixos/options.json $out"

this generates the NixOS module documentation JSON, and home-manager json doc can be generated with nix-build -E '(import <home-manager> {}).docs.json'

[elkowar]

Also: we should look at parsing the XML the nix documentation is generated from, to get documentation for the stdenv and lib functions. The additional information and cleanness available there would also allow me to provide much better autocompletion (as looking at the documentation gives me a full namespace that i can mach against, whereas the comments don't allow for that)

[elkowar]

Both of this was implemented!