home-manager: add documentation to root `default.nix`

This adds documentation related packages to default.nix to allow
building documentation separately from building a Home Manager
configuration.

default.nix

@@ -1,6 +1,12 @@
 { pkgs ? import <nixpkgs> { } }:
 
 rec {
+  docs = with import ./doc { inherit pkgs; }; {
+    html = manual.html;
+    manPages = manPages;
+    json = options.json;
+  };
+
   home-manager = pkgs.callPackage ./home-manager { path = toString ./.; };
 
   install =

doc/default.nix

@@ -1,6 +1,7 @@
-{
+{ pkgs
+
 # Note, this should be "the standard library" + HM extensions.
-lib, pkgs }:
+, lib ? import ../modules/lib/stdlib-extended.nix pkgs.lib }:
 
 let