docs: document installation, secrets and setup steps

nixos/modules/dns.nix

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