Docs: Add 'Optional' to installation section to fix an issue (#28902)

Problem:
"Optional" object is used in one example without importing, which raises
the following error when copying the example into IDE or Jupyter Lab

![image](https://github.com/user-attachments/assets/3a6c48cc-937f-4774-979b-b3da64ced247)

Solution:
Just importing Optional from typing_extensions module, this solves the
problem!

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

docs/docs/how_to/structured_output.ipynb

@@ -165,6 +165,8 @@
     }
    ],
    "source": [
+    "from typing import Optional\n",
+    "\n",
     "from typing_extensions import Annotated, TypedDict\n",
     "\n",
     "\n",
@@ -206,10 +208,10 @@
     {
      "data": {
       "text/plain": [
-        "{'setup': 'Why was the cat sitting on the computer?',\n",
-        " 'punchline': 'Because it wanted to keep an eye on the mouse!',\n",
-        " 'rating': 7}"
-       ]
+       "{'setup': 'Why was the cat sitting on the computer?',\n",
+       " 'punchline': 'Because it wanted to keep an eye on the mouse!',\n",
+       " 'rating': 7}"
+      ]
      },
      "execution_count": 4,
      "metadata": {},