Add cookbook on RAG observability with LangChain and MLflow

[daniellok-db]

Summary

Briefly describe the changes and the goal of this PR. Make sure the PR title summarizes the changes effectively.

This PR adds a new cookbook demonstrating an observability tool (MLflow Tracing) when building a RAG chain with LangChain and OpenAI.

Motivation

Why are these changes necessary? How do they improve the cookbook?

There are a few cookbooks on RAG at the moment, but none of them have observability as a focus. This notebook links together a few open source libraries to showcase new debugging tools (e.g. tracing) and how they can be helpful when building a RAG application.

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