add links to summary sections

ch04/01_main-chapter-code/ch04.ipynb

@@ -1480,6 +1480,17 @@
     "- Note that the model is untrained; hence the random output texts above\n",
     "- We will train the model in the next chapter"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "a35278b6-9e5c-480f-83e5-011a1173648f",
+   "metadata": {},
+   "source": [
+    "## Summary and takeaways\n",
+    "\n",
+    "- See the [./gpt.py](./gpt.py) script, a self-contained script containing the GPT model we implement in this Jupyter notebook\n",
+    "- You can find the exercise solutions in [./exercise-solutions.ipynb](./exercise-solutions.ipynb)"
+   ]
   }
  ],
  "metadata": {