docs: extract setup example into .nix file and include

That way we get linting of the code for free.

docs/setup-guide.rst

@@ -57,42 +57,8 @@ though there are more possible options (see the `NixOS Mailserver
 options documentation <options.html>`_), these should be the most
 common ones.
 
-.. code:: nix
-
-   { config, pkgs, ... }: {
-     imports = [
-       (builtins.fetchTarball {
-         # Pick a release version you are interested in and set its hash, e.g.
-         url = "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/archive/nixos-25.11/nixos-mailserver-nixos-25.11.tar.gz";
-         # To get the sha256 of the nixos-mailserver tarball, we can use the nix-prefetch-url command:
-         # release="nixos-25.11"; nix-prefetch-url "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/archive/${release}/nixos-mailserver-${release}.tar.gz" --unpack
-         sha256 = "0000000000000000000000000000000000000000000000000000";
-       })
-     ];
-
-     mailserver = {
-       enable = true;
-       stateVersion = 3;
-       fqdn = "mail.example.com";
-       domains = [ "example.com" ];
-
-       # A list of all login accounts. To create the password hashes, use
-       # nix-shell -p mkpasswd --run 'mkpasswd -sm bcrypt'
-       loginAccounts = {
-         "user1@example.com" = {
-           hashedPasswordFile = "/a/file/containing/a/hashed/password";
-           aliases = ["postmaster@example.com"];
-         };
-         "user2@example.com" = { ... };
-       };
-
-       # Use Let's Encrypt certificates. Note that this needs to set up a stripped
-       # down nginx and opens port 80.
-       certificateScheme = "acme-nginx";
-     };
-     security.acme.acceptTerms = true;
-     security.acme.defaults.email = "security@example.com";
-   }
+.. literalinclude:: ./setup-example.nix
+   :language: nix
 
 After a ``nixos-rebuild switch`` your server should be running all
 mail components.