akka streams docs: graphs

WIP on graphs parts of docs.

refs https://github.com/akka/akka/issues/16549
The trailing dot is easily missed, especially since it does not line up. I’d rather move it to the next line:
~~~scala
     b.addEdge(in, broadcast)
      .addEdge(broadcast, f1, merge)
      .addEdge(broadcast, f2, merge)
      .addEdge(merge, f3, out)
~~~