add nixos frontend to simple architecture diagram

architecture-docs/2025-05-architecture-replaced-by-mermaid.dot

@@ -1,4 +1,4 @@
-// usage: nix-shell -p graphviz --command dot -T png -o architecture.png architecture.dot
+// usage: nix-shell -p graphviz --command 'dot -T png -o architecture.png architecture.dot'
 
 digraph FediversityArchitecture {
   graph[fontname="Liberation Sans"];
@@ -7,11 +7,13 @@ digraph FediversityArchitecture {
   subgraph cluster_frontends {
     label="front-ends";
 
+    nix[label="NixOS module"];
     tofu[label="OpenTofu CLI"];
     panel[label="FediPanel"];
     protagio[label="NixPanel"];
   }
 
+  nix -> core;
   tofu -> core;
   panel -> core;
   protagio -> core;