fix/multipage demo (whitphx#1127)

* Add intro at streamlit_app.py

* Rename pages/*.py adding the prefix numbers

* Rename streamlit_app.py to home.py

* Apply black

Author: whitphx

streamlit_app.py home.py

@@ -5,6 +5,10 @@
 logger = logging.getLogger()
 
 st.title("streamlit-webrtc demo!")
+st.info(
+    """👈 Select the demo
+"""
+)
 
 
 if __name__ == "__main__":