add multiple apps to reference

[gmanchon]

from Clément Lelièvre #502

https://blog.streamlit.io/introducing-multipage-apps/

[ssaunier]

What's the appetite on that one? Is it just adding a dummy file to quickly show students that it's possible? How do they use this repo? Do they clone it?

cc @krokrob

[krokrob]

delegating it to pedagogical committee

[gmanchon]

this is the code for https://streamlit.lewagon.ai/
we could add a file to demo the feature
or refacto the app to take advantage of it