akka-streams docs - quickstart

Loads of things to do still, but first wanted some feedback on the quickstart. If this is the right style we should be aiming for. 

Most likely needs more links into the in depth parts of the docs, but as they don't exist yet that was though ;-)

And important note - these docs still use the old layout, we need to fix that as well.

refs https://github.com/akka/akka/issues/16483
Oh, seems I can simplify here - thanks (remove line 59, and move 55 into Example1 - can rename example1 as well)

Reason was: due to extending AkkaSpec I already have an implicit actorSystem in scope. If it would be in the `Example1` scope I'd get an ambigious implicit error on line 59 - creating the flow materializer.