docs: update Flakes example

and add a small example how to deploy with nixos-rebuild.

docs/flakes.nix

@@ -0,0 +1,35 @@
+{
+  description = "NixOS configuration";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11-small";
+
+    simple-nixos-mailserver.url = "gitlab:simple-nixos-mailserver/nixos-mailserver/nixos-25.11";
+    simple-nixos-mailserver.inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  outputs =
+    {
+      nixpkgs,
+      simple-nixos-mailserver,
+      ...
+    }:
+    {
+      nixosConfigurations = {
+        hostname = nixpkgs.lib.nixosSystem {
+          system = "x86_64-linux"; # or aarch64-linux
+          modules = [
+            simple-nixos-mailserver.nixosModule
+            {
+              mailserver = {
+                enable = true;
+
+                # Check the setup guide if you have no idea how to continue
+                # from here!
+              };
+            }
+          ];
+        };
+      };
+    };
+}

docs/flakes.rst

@@ -1,30 +1,18 @@
-Nix Flakes
-==========
+Flakes
+======
 
-If you're using `flakes <https://wiki.nixos.org/wiki/Flakes>`__, you can use
-the following minimal ``flake.nix`` as an example:
+To use NixOS mailserver `Nix flakes`_, the following minimal ``flake.nix`` can
+serve as an example to get started:
 
-.. code:: nix
+.. _Nix flakes: https://wiki.nixos.org/wiki/Flakes
 
-   {
-     description = "NixOS configuration";
+.. literalinclude:: ./flakes.nix
+   :language: nix
 
-     inputs.simple-nixos-mailserver.url = "gitlab:simple-nixos-mailserver/nixos-mailserver/nixos-20.09";
 
-     outputs = { self, nixpkgs, simple-nixos-mailserver }: {
-       nixosConfigurations = {
-         hostname = nixpkgs.lib.nixosSystem {
-           system = "x86_64-linux";
-           modules = [
-             simple-nixos-mailserver.nixosModule
-             {
-               mailserver = {
-                 enable = true;
-                 # ...
-               };
-             }
-           ];
-         };
-       };
-     };
-   }
+Lock the inputs and deploy the system closure:
+
+.. code-block:: console
+
+   nix flake lock
+   nixos-rebuild --target-host root@mail.example.com --flake .#hostname switch