Additional stream operators to include in overview

There are a number of stream operators not yet documented at https://doc.akka.io/docs/akka/current/stream/operators/index.html .

The most current list of not-yet-documented operators can be found at https://github.com/akka/akka/blob/master/project/StreamOperatorsIndexGenerator.scala#L78 . Some probably can remain undocumented, those should be moved to the 'ignore list at https://github.com/akka/akka/blob/master/project/StreamOperatorsIndexGenerator.scala#L121

To document an operator:

• Optionally, add a comment to this issue 'claiming' the operator
• Remove it from the 'pending' list
• Add the markdown file to /akka-docs/src/main/paradox/stream/operators. The convention is Classname/operatorname.md, with the exception of operators that are available on both Sources and Flows. Look at the other files for examples.
• run sbt paradox to test generating the documentation