[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]

Netlify deploy preview

• docs/data/material/components/slider/slider.md

Bundle size report

No bundle size changes (Toolpad) No bundle size changes

Generated by :no_entry_sign: dangerJS against 6acf57f12b29f4241f2baf451d968f88fb414265

[oliviertassinari]

maybe other demos need it, not sure.

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