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/default.nix

@@ -0,0 +1,16 @@
+# System-related modules
+{ ... }:
+
+{
+  imports = [
+    ./boot
+    ./docker
+    ./documentation
+    ./language
+    ./nix
+    ./packages
+    ./podman
+    ./printing
+    ./users
+  ];
+}