[slider][docs] Polish Music player demo

[oliviertassinari]

A quick polish on #43743. I thought this would look better and be denser, using less vertical space:

Before

After

This docs option is not used so often on Material UI. We use it a lot with the MUI X Data Grid though, e.g. https://mui.com/x/react-data-grid/#mit-license-free-forever

[mui-bot]

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[aarongarciah]

Nice! TIL about bg: inline.

I'll take a look and see if we can get rid of the top margin of the bottom bar when bg: inline is present, but maybe other demos need it, not sure.

Also, it'd be nice that the demo follows the border radius of the container:

[oliviertassinari]

maybe other demos need it, not sure.

For the data grid demos linked in the PR description, I doubt it would work without 😁