+doc akka streams docs, flowGraphs
Merge pull request #16555 from ktoso/docs-streams-graphs

akka streams docs: graphs
!str #16484 #16392 Remove String identifier of junctions

* descriptive name can still be defined with OperationAttributes.name
* revisit all factory methods (apply/create) and implement and document
  them in a consistent manner
!str #16400 Add Java API for StreamTcp

* move io.StreamTcp to scaladsl.StreamTcp
* add javadsl.StreamTcp that delegates to scaladsl implementation
* move impl
* add java test
Merge pull request #16562 from akka/wip-16484-removed-junction-name-patriknw

!str #16484 #16392 Remove String identifier of junctions
Merge pull request #16550 from akka/wip-16400-javadsl-stream-io-patriknw

!str #16400 Add Java API for StreamTcp
+doc #15239: Stream cookbook
fixed merge conflicts, fixed reducebykey