Doc: Clarify upgrade path for new integration plugins + configurable settings

[ppf2]

With the release of the new integration plugins, questions also started to come up from the field :)

It will be helpful to include a paragraph to indicate the following in the documentation areas for these integration plugins (Kafka in this example):

• When going from pre-7.5 to 7.5+, it is not necessary to install or update to use the new integration plugins. The new integration plugins simply contain the previous plugins, e.g.

logstash-integration-kafka (10.0.0)
 ├── logstash-input-kafka
 └── logstash-output-kafka

When the user installs Logstash 7.5, they will already get the integration plugins and not have to worry about having to remove the individual input and output plugins before installing the new integration plugin, etc..

• The current docs (e.g., https://www.elastic.co/guide/en/logstash/current/plugins-integrations-kafka.html) do not provide any configuration information - which is intended because existing logstash-input-kafka and logstash-output-kafka configurations should continue to function as-is; it is not necessary to modify the Logstash configuration just to use/reference the integration plugin. I will suggest stating this in the doc and cross linking to the individual input and output plugin documentation areas for users who are looking for settings for the integration plugins.

Thx!