chore: add reference docs MCP (#35737)

.mcp.json

@@ -3,6 +3,10 @@
     "docs-langchain": {
       "type": "http",
       "url": "https://docs.langchain.com/mcp"
+    },
+    "reference-langchain": {
+      "type": "http",
+      "url": "https://reference.langchain.com/mcp"
     }
   }
-}
+}