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
Do we wan't to have a big fat file for this? I assumed that the different parts are on different pages, each starting with a motivating sample.