Additional sets of stream operators to include in overview

The Akka Stream overview page at https://doc.akka.io/docs/akka/current/stream/operators/index.html is generated from the Scala sources.

There are some Scala files that are not scanned for operators yet:

• #28644 akka.stream.{scaladsl,javadsl}.Compression
• akka.stream.{scaladsl,javadsl}.CoupledTerminationFlow
• akka.stream.{scaladsl,javadsl}.Framing
• akka.stream.{scaladsl,javadsl}.JsonFraming
• akka.stream.{scaladsl,javadsl}.MergeHub
• akka.stream.{scaladsl,javadsl}.PartitionHub
• akka.stream.{scaladsl,javadsl}.StreamRefs
• akka.stream.{scaladsl,javadsl}.Tcp

Sets can be added in separate PR's. To add a set, pick one from the list above and add a comment to this issue to 'claim' it. Add it to the list at https://github.com/akka/akka/blob/master/project/StreamOperatorsIndexGenerator.scala#L135 and add markdown documentation for at least one of the operators. The others you can add to the list of 'pending' operators to document at https://github.com/akka/akka/blob/master/project/StreamOperatorsIndexGenerator.scala#L78 (keys are class names, values are sequences of operator names).

If the operator you documented fits in an existing category, make sure to add a @ref to that category at the end of the first section and it should appear automatically.

If the operator does not fit in an existing category, add a new category to https://github.com/akka/akka/blob/master/project/StreamOperatorsIndexGenerator.scala#L22 , with a category summary in https://github.com/akka/akka/tree/master/akka-docs/src/main/categories .

Run 'sbt paradox' to test.