docs: add MySQLVectorStore reference notebook

[jackwotherspoon]

Adding reference notebook for basic usage of MySQLVectorStore.

Rendered notebook: https://github.com/googleapis/langchain-google-cloud-sql-mysql-python/tree/vector-store-docs/docs/vector_store.ipynb

[totoleon]

Tested in colab and it's working fine.

• For the index operations, we need to manually grant the IAM user permissions

GRANT EXECUTE ON PROCEDURE mysql.create_vector_index TO 'hailongli'@'%';
GRANT EXECUTE ON PROCEDURE mysql.alter_vector_index TO 'hailongli'@'%';
GRANT EXECUTE ON PROCEDURE mysql.drop_vector_index TO 'hailongli'@'%';
GRANT SELECT ON mysql.vector_indexes TO 'hailongli'@'%';

• nit: do we need to add a cleanup function at the end to drop the test table?

[jackwotherspoon]

• For the index operations, we need to manually grant the IAM user permissions

@totoleon I'll test today with built-in user and add a note to the index section if need be.

nit: do we need to add a cleanup function at the end to drop the test table?

I don't think we need to cleanup the table, this is just the reference documentation for our APIs and methods. The other vector store langchain integrations do not demonstrate dropping the table/cleanup.