Doc: Expand integrations docs to allow for more use cases

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. While `logstash-input-elastic_agent` will be the preferred input for the majority of use cases, we should add info about other possibilities.

cc:/ @flexitrev

Start with the integrations topic at https://www.elastic.co/guide/en/logstash/master/ea-integrations.html and review its current explanation of logstash-filter-elastic_integration and logstash-input-elastic_agent. Identify additional supported use cases and document them clearly; done means the topic covers more than the basic preferred-input workflow.

Written by the indexing model from the issue text.