WIP Initial draft of akka-streams QuickStart and docs

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
I couldn't find good wording for it. It's basically because it would lead to unexpected behaviours  as the semantics are a bit different? I remember trying to persuade Heiko at some point why we're not using the word `flatMap` and he wouldn't get persuaded easily ;-)