[Feature/Doc request] Please add an example for the chatbot use case

[jpzhangvincent]

It would be good to add an example on how to deploy the API for the ConversationalRetrievalChain that keeps tracks of memory dynamically or using ConversationBufferWindowMemory.

Reference:

1. https://github.com/hwchase17/chat-langchain (some of the code is outdated)
2. https://github.com/ajndkr/lanarky/blob/main/examples/app/conversational_retrieval.py