Universal: docs: Engines need better docs, demos and guidance

[Toxicable]

As pointed out by @wardbell here https://github.com/angular/angular/pull/18707#discussion_r138766009
We should address the points raised, most of which are simply due to a lack of docs, examples and demos.

[MarkPieszak]

Docs (.MD files even), or updates to the universal.angular.io would probably be best since it's a slippery slope if we start demo'ing everything. The starter starts turning into the kitchen-sink again 😆

[Toxicable]

I think we need to point universal.angular.io here instead of duplicating content.
And each module should have it's own demo showing it's specific features, no kitchen sink here, these demo's could also be apart of intergration/e2e testing once we implement that.
Although in saying that I also think that each Engine should have the same features, as I'm sort of doing here https://github.com/angular/universal/pull/780