refactor: move nixos-manual to reference/ + graphifyignore

reference/nixos-manual/development/importing-modules.section.md

@@ -0,0 +1,23 @@
+# Importing Modules {#sec-importing-modules}
+
+Sometimes NixOS modules need to be used in configuration but exist
+outside of Nixpkgs. These modules can be imported:
+
+```nix
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+{
+  imports = [
+    # Use a locally-available module definition in
+    # ./example-module/default.nix
+    ./example-module
+  ];
+
+  services.exampleModule.enable = true;
+}
+```