Update documentation with pip install langchain-community

[jmferrer]

In streamlit documentation is used this file that is OK: https://github.com/streamlit/llm-examples/blob/main/pages/3_Langchain_Quickstart.py
And the requirements are OK.

BUT the documentation is broken because the langchain-community is missed: https://docs.streamlit.io/develop/tutorials/llms/llm-quickstart

I'm sorry if this is not the right place to file the issue but I couldn't find another place.