docs: redirect /sandboxing to /gateway/sandboxing

docs/docs.json

@@ -229,6 +229,14 @@
       "source": "/sandbox/",
       "destination": "/cli/sandbox"
     },
+    {
+      "source": "/sandboxing",
+      "destination": "/gateway/sandboxing"
+    },
+    {
+      "source": "/sandboxing/",
+      "destination": "/gateway/sandboxing"
+    },
     {
       "source": "/AGENTS.default",
       "destination": "/reference/AGENTS.default"