[docs] Create SliderUnstyled docs

[michaldudak]

Moved the SliderUnstyled docs to Base and created more unstyled demos based on the Material UI ones.

Preview: http://deploy-preview-31850--material-ui.netlify.app/base/react-slider

[mui-bot]

No bundle size changes

Generated by 🚫 dangerJS against 47aac7f

[mnajdova]

Suggested change

	Sliders show a range of values along a bar, from which users may select a single value.
	Sliders show a range of values along a bar, from which users may select a single value, or a range.

Not sure how to phrase that users can select a range too, for example from the sliders with two thumbs.

[mnajdova]

We should improve the hover styles:

Also, we need a focus visible styles on the thumb, there is not indication that it is focused.

[samuelsycamore]

What does "thumb" refer to here? I don't see this word used anywhere else in the document. Is this the accepted term for the control itself?

[michaldudak]

Good point - we haven't introduced this term before. It may be worth having an image explaining the anatomy of a slider, similar to https://material.io/components/sliders#anatomy. Other controls would benefit from it too.
@danilo-leal, would you agree?

[danilo-leal]

Absolutely. Similar to the customization examples, I reckon we could add these in bulk in a separate PR once the first iteration of each component docs is done!

[mnajdova]

Looks good, I would recommend in the next pass to change a bit the look of the focus visible and selected styles of the thumbs, as they look a lot like Material Design :)