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
Add section: "How to read these docs" (or something similar)
It should be roughly:
 - read the quickstart to get a feel
 - (optional) read the design statement
 - (optional) look at the cookbook probably parallelly while reading the main docs as supplementary material
 - the other sections can be read sequentially, each digging deeper into advanced topics