Rethinking about the Sidebar implement

[luubinhan]

The way we implement the Sidebar component I think it still have room to improve

With the overlay Sidebar, why do we need to put the children component inside <Sidebar.Pusher />, in somecase it's not nessesary and raise some issue, beause sometimes we could put our content inside <Sidebar.Pusher />

I found it more straight forwards, and more independent with
https://material-ui.com/demos/drawers/
or
https://developer.microsoft.com/en-us/fabric#/components/panel

[welcome]

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[layershifter]

I will close this issue because we use SUI CSS now and current markup is required. However, I agree it's not friendly. Possible we can make changes in markup in v2.

BTW, the Sidebar component will receive some improvements in near future, see #2845.