First bits of akka-http-core Scala server documentation for review

/cc @sirthias 
Yes, we can probably remove low-level almost completely as there will be no other server than this one.

> Also, I haven't seen any numbers on top of our streams , so how much high-performance is there right now?

I have some numbers but I think the point was to formulate our high aspirations more than the current state which still has optimization potential... That said, I'd wouldn't object if you wanted to tone down this claim until we have nice numbers to show to keep expectations realistic for now. Any suggestions?