Integrate List component with Dialog in docs and screenshot tests

[kfranqueiro]

Now that #3546 is in, the dialog screenshot test pages should instantiate MDC List when applicable, and documentation should be augmented to include an example of this.

I've already started working on this, and it additionally looks like we'll probably want to add keydown enter/space handling for data-mdc-dialog-action to cover all scenarios. Otherwise currently you need to use a div + a elements (plus fix a bug in List, which I'll PR separately) in order to capture space/enter on the list in the Simple Dialog variant (by making the list items fire clicks), which feels a little awkward.