Add an schematic of the graph to the docs

From the [quick start](https://langchain-ai.github.io/langgraph/tutorials/introduction/#part-1-build-a-basic-chatbot)
```
from IPython.display import Image, display

try:
    display(Image(graph.get_graph().draw_mermaid_png()))
except Exception:
    # This requires some extra dependencies and is optional
    pass
```