Create Slider.mdx (#26149)

add simple documentation for slider component

packages/grafana-ui/src/components/Slider/Slider.mdx

+import { Meta, Props } from '@storybook/addon-docs/blocks';
+import { Slider } from './Slider';
+
+<Meta title="MDX|Slider" />
+
+# Slider
+
+The `Slider` component is an input element where users can manipulate one or two values on a one-dimensional axis. 
+
+`Slider` can be implemented in horizontal or vertical orientation. You can set the default starting value(s) for the slider with the `value` prop. 
+
+<Props of={Slider} />