A basic component for the app header. The header outputs breadcrumbs via the `sections` prop (required) and access to the activity panel. It also sets the document title. The Header component used in each section automatically fills into the "header" slot defined in `<Layout />`. We're using [react-slot-fill](https://github.com/camwest/react-slot-fill) to avoid a duplicated `div` wrapper from Gutenberg's implementation.
*`sections` (required): Used to generate breadcrumbs. Accepts a single items or an array of items. To make an item a link, wrap it in an array with a relative link (example: `[ '/analytics', __( 'Analytics', 'wc-admin' ) ]` ).