Add streamlit + pne.chat simple chat app use cases

[Undertone0809]

🚀 Feature Request

Just build a simple chatbot by streamlit and pne.chat

Functions:

• only chat
• select different models
• input env and key
• stream output

Output

• a example code in examples

Example directories:

• examples/streamlit-chatbot/app.py

• examples/streamlit-chatbot/requirements.txt

• examples/streamlit-chatbot/README.md

• a docs

Why do it?

Let developers see how gradio combines with pne.chat.

[Undertone0809]

If you finish this, you can do #626 at the same time.