docs: document installation, secrets and setup steps

nixos/modules/nginx.nix

@@ -1,4 +1,4 @@
-{ config, lib, ... }:
+{ config, ... }:
 let
   inherit (config.networking) fqdn;
 
@@ -14,11 +14,6 @@ let
   };
 in
 {
-  networking.firewall.allowedTCPPorts = [
-    80
-    443
-  ];
-
   services.nginx = {
     enable = true;
 
@@ -38,4 +33,9 @@ in
       locations."/" = mkProxy config.services.gitea.settings.server.HTTP_PORT;
     };
   };
+
+  networking.firewall.allowedTCPPorts = [
+    80
+    443
+  ];
 }