Scalafix followups: Move doc specs to doc/jdoc package

https://github.com/akka/akka/pull/26019#pullrequestreview-181636601 introduced scalafix to remove unused imports. It is not enabled on compile and can remove imports that are included in specs for docs.

To followup:

• Ignore files in doc/jdoc package
• Move any doc specs to the doc for Scala and jdoc package so that imports are necessary e.g. FlowZipSpec is in the akka.stream.scaladsl and is used for docs. The unit test parts should remain but the docs part should be in a new file in the docs package do that imports from akka.stream.scaladsl are required
• Test how fast it is to enable on compile and decide if we want it enabled by default