akka streams docs: graphs

WIP on graphs parts of docs.

refs https://github.com/akka/akka/issues/16549
Defuse common pitfalls:
 - forgetting to explicitly give the port types on junctions
 - junction reference equality defines graph node equality in the graph (i.e. the same Zip instance refers to the same location in the graph)
 - Flows are reusable inside the graph (Flow equality, reference equality does not imply edge equality), parallel edges are possible