docs[minor]: LCEL rewrite of chatbot use-case (#16414)
CC @baskaryan @hwchase17
TODO:
- [x] Draft of main quickstart
- [x] Index intro page
- [x] Add subpage guide for Memory management
- [x] Add subpage guide for Retrieval
- [x] Add subpage guide for Tool usage
- [x] Add LangSmith traces illustrating query transformation
