refactor: simplify scroller basic example

[sissbruecker]

Simplify the basic Scroller example to just show a card with some Markdown content. This makes the example work out of the box with both Lumo and Aura.

[vursen]

The overflow indicators look a bit strange in this example, do we actually need them? Otherwise LGTM.

[jouni]

The overflow indicators look a bit strange in this example

Happens only in the Flow example, where there’s an extra <div style="display: contents"> element wrapping the <vaadin-scroller>. That prevents this selector from matching: https://github.com/vaadin/web-components/blob/b52505a031db01d2aba91ec4126d6cf98c9becd3/packages/card/src/styles/vaadin-card-base-styles.js#L252-L260

Those should actually be updated to use the --vaadin-scroller-padding-inline property, and we can then remove the selectors targeting the scroller pseudo elements.