Added subzero to menu

Author: JanBobolz

_data/navigation.yaml

@@ -11,6 +11,10 @@ docs:
       url: /getting-started/protocols-tutorial.html
     - title: "ElGamal Tutorial"
       url: /getting-started/implement-elgamal.html
+- title: Tools
+  children:
+    - title: Subzero ZK Compiler
+    url: https://cryptimeleon.org/subzero      
 - title: Documentation
   children:
     - title: "How do I ...?"