docs: configure ACME HTTP-01 with nginx in setup example

docs/setup-example.nix

@@ -16,12 +16,14 @@
   security.acme = {
     acceptTerms = true;
     defaults.email = "security@example.com";
-    certs.${config.mailserver.fqdn} = {
-      # Further setup required, check the manual:
-      # https://nixos.org/manual/nixos/stable/#module-security-acme
-    };
   };
 
+  # Allow incoming HTTP connections
+  networking.firewall.allowedTCPPorts = [ 80 ];
+
+  # Enable ACME HTTP-01 challenge with nginx
+  services.nginx.virtualHosts.${config.mailserver.fqdn}.enableACME = true;
+
   mailserver = {
     enable = true;
     stateVersion = 3;