Manually fixed path to correct lightning documentation

Auto intersphinx would try to access "https://lightning.ai/lightning-docs/objects.inv" and would be
redirected to "https://lightning.ai/docs/app/stable/objects.inv" instead
of "https://lightning.ai/docs/pytorch/stable/objects.inv"

doc/catalog.json

@@ -5,5 +5,13 @@
       "latest": "https://clapper.readthedocs.io/en/latest/"
     },
     "sources": {}
+  },
+  "lightning": {
+    "versions": {
+      "stable": "https://lightning.ai/docs/pytorch/stable/"
+    },
+    "sources": {
+      "environment": "lightning"
+    }
   }
 }