docs: Add memgraph notebook (langchain-ai#9448)

- Description: added graph_memgraph_qa.ipynb which shows how to use LLMs
to provide a natural language interface to a Memgraph database using
[MemgraphGraph](langchain-ai#8591)
class.
- Dependencies: given that the notebook utilizes the MemgraphGraph
class, it relies on both this class and several Python packages that are
installed in the notebook using pip (langchain, openai, neo4j,
gqlalchemy). The notebook is dependent on having a functional Memgraph
instance running, as it requires this instance to establish a
connection.

Author: AlexIchenskiy