[dialog] Document using scroller for creating accessible scrollable content

[web-padawan]

Describe your motivation

Part of vaadin/platform#2785

Ensure content area is scrollable by keyboard (or provide documentation for how to make the content area scrollable depending on the content)

Based on discussion we had, it seems like the easiest way to address this would be to provide example of using vaadin-scroller component inside of the dialog: scroller has tabindex which makes it possible to scroll with keyboard.

Describe the solution you'd like

Add example of using vaadin-scroller for scrollable content inside of the vaadin-dialog.

[web-padawan]

See also #127 which can be closed once this issue is resolved.