docs: document installation, secrets and setup steps

nixos/modules/dns.nix

@@ -80,17 +80,21 @@ in
       };
     };
 
-    networking.firewall.allowedTCPPorts = [ config.services.bind.listenOnPort ];
+    environment.persistence."/persist".directories = [
-    networking.firewall.allowedUDPPorts = [ config.services.bind.listenOnPort ];
-
-    environment.persistence."/persist" = {
-      directories = [
       {
         directory = "/var/dns";
         group = "named";
         user = "named";
       }
     ];
+
+    networking.firewall =
+      let
+        ports = [ config.services.bind.listenOnPort ];
+      in
+      {
+        allowedTCPPorts = ports;
+        allowedUDPPorts = ports;
       };
   };
 }