[FEA] Deprecate app switcher to new form

[lmeyerov]

Streamlit now supports multipage apps, so we should switch to that to avoid confusion: https://blog.streamlit.io/introducing-multipage-apps/

This will take a bit of research first:

• deprecating handle legacy use: let's not break things, such as by an up-converter with a warning?
• transforming legacy demos
• new docs
• handling scale: the new UI seems to do a list instead of a dropdown, which seems like a problem for use cases like ours where we have many, so this may also be difficult