Add README, pydoc and notebook

[cbornet]

This pull request introduces the LangChain integrations for using Vectorize, including updates to the documentation, code, and an example notebook. The most important changes are grouped by theme below.

Documentation and Setup:

• Added a new README.md section for installation and setup instructions for the langchain-vectorize package.
• Updated the VectorizeRetriever class docstring to include detailed setup, initialization arguments, and usage examples.

Code Enhancements:

• Imported VectorizeRetriever in the __init__.py file and added it to the __all__ list for easier access.

Example Notebook:

• Added a new Jupyter notebook langchain_retriever.ipynb demonstrating how to use the LangChain Vectorize retriever, including setup, configuration, and usage examples.

Usage Improvements:

• Modified the invoke method in VectorizeRetriever to include a more descriptive query example.

[cbornet]

@nicoloboschi do you have time to review ?

[nicoloboschi]

LGTM