website: use shadow DOM to render DayPicker in the examples

[gpbl]

In the website we use Sandpack to render DayPicker because Docusaurus overrides the default style from DayPicker.

A workaround for this solution is to use shadow DOM (e.g. via https://www.npmjs.com/package/react-shadow) for a proper style encapsulation. This would provide an optimal testing environment when developing DayPicker.