Doc: Working with Logstash Plugins on ECK (#7702)

Fixes: elastic/ingest-dev#3023

Additional iterations based on excellent work in @robbavey's feature branch.
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Co-authored-by: Rob Bavey <rob.bavey@elastic.co>
Co-authored-by: kaisecheng <69120390+kaisecheng@users.noreply.github.com>
Co-authored-by: Peter Brachwitz <peter.brachwitz@gmail.com>