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
explicit is misleading, it might be interpreted as the user of the library needs to send explicit backpressure signals. Maybe explicit internal backpressure signals?