docs: add formal verification page to Mintlify navigation

docs/docs.json

@@ -956,6 +956,7 @@
           "gateway/doctor",
           "gateway/logging",
           "gateway/security",
+          "gateway/security/formal-verification",
           "gateway/sandbox-vs-tool-policy-vs-elevated",
           "gateway/sandboxing",
           "gateway/troubleshooting",