docs: document installation, secrets and setup steps

nixos/common.nix

@@ -93,8 +93,6 @@ in
   networking = {
     networkmanager.enable = true;
 
-    firewall.enable = true;
-
     extraHosts =
       let
         subdomains = [
@@ -113,6 +111,14 @@ in
           ::1 ${host}
         ''
       ) "" subdomains;
+
+    firewall = {
+      enable = true;
+
+      allowedTCPPorts = [
+        22
+      ];
+    };
   };
 
   virtualisation.podman.enable = true;

nixos/openssh.nix

@@ -10,6 +10,4 @@
       PermitRootLogin = "no";
     };
   };
-
-  networking.firewall.allowedTCPPorts = [ 22 ];
 }