Doc: Add filter-elastic_integration tutorial for extending integrations

[karenzone]

Topic: https://www.elastic.co/guide/en/logstash/master/ea-integrations.html

The current topic provides users with the basic info and concepts needed to use logstash-filter-elastic_integration in their pipelines.
Next step: Add a tutorial.

[bmorelli25]

Need developer resources before we can move forward.

[karenzone]

cc:/ @robbavey

[mashhurs]

I have reviewed/validated the #15932 and suggested simplifications on top of. We are good to go with this. 🚀