RAG example

[sophiamyang]

Hi @MarcSkovMadsen, thanks so much for improving this example!!
https://github.com/holoviz-topics/panel-chat-examples/blob/main/docs/examples/langchain/langchain_pdf_assistant.py

I have a question: this example relies on this other file in the repo (from panel_chat_examples import EnvironmentWidgetBase). People can't just copy and paste this example and get the app running. I guess it's okay but might be difficult for new users to figure everything out. What do you think? @MarcSkovMadsen @ahuang11