[FR]: Add @ExperimentalSparkApi opt-in note to Sliders.md

Describe the problem

Both Slider and RangeSlider are annotated @ExperimentalSparkApi in source but the documentation does not mention this. Users who follow the docs get a compile error without the opt-in annotation.

Describe the solution

Add a callout to Sliders.md:

> [!IMPORTANT]
> Both `Slider` and `RangeSlider` require `@OptIn(ExperimentalSparkApi::class)`.

Additional context

Priority: Medium. Effort: XS.