test(material/slider): improve slider demo

[wagnermaciel]

No description provided.

[mmalerba]

LGTM - probably shouldn't be "feat()" though

[wagnermaciel]

@mmalerba Would docs() be more appropriate? Also, here's a quick visual for what the demo looks like now

[mmalerba]

I can't remember if docs shows up in the changelog or not (we probably don't want this to show up since its not public facing docs), test would be a good option too, since its related to manual testing

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