nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/system/documentation/default.nix

@@ -0,0 +1,43 @@
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.my.system.documentation;
+in
+{
+  options.my.system.documentation = with lib; {
+    enable = my.mkDisableOption "Documentation integration";
+
+    dev.enable = my.mkDisableOption "Documentation aimed at developers";
+
+    info.enable = my.mkDisableOption "Documentation aimed at developers";
+
+    man = {
+      enable = my.mkDisableOption "Documentation aimed at developers";
+
+      linux = my.mkDisableOption "Linux man pages (section 2 & 3)";
+    };
+
+    nixos.enable = my.mkDisableOption "NixOS documentation";
+  };
+
+  config = lib.mkIf cfg.enable {
+    documentation = {
+      enable = true;
+
+      dev.enable = cfg.dev.enable;
+
+      info.enable = cfg.info.enable;
+
+      man = {
+        enable = cfg.man.enable;
+        generateCaches = true;
+      };
+
+      nixos.enable = cfg.nixos.enable;
+    };
+
+    environment.systemPackages = with pkgs; lib.optionals cfg.man.linux [
+      man-pages
+      man-pages-posix
+    ];
+  };
+}