Make sections in navigation menu collapsable

[mischlrebl]

Checklist

• [X] I have searched the existing issues for similar feature requests.
• [X] I added a descriptive title and summary to this issue.

Summary

A project with several/many sections and pages shows at start and all the time every entity. To minimize that view for the user and to focus, it should be possible to collapse sections (with its pages below).

Why?

The app should be able to leave away, and give the user the ability to do so by itself, what is not immediately necessary or important

How?

add an optional parameter for sections in pages: collapse(true/false) which declares the inital state on load

behaviour:

1. if the parameter itself is given, show an icon after the header title
2. the icon depends on the value:
   1. false: ▲
   2. true: ▼

Additional Context

take a look on the menu on the left on https://docs.streamlit.io/

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