[APM] Update terminology

[bmorelli25]

Elastic APM consists of four components: APM agents, Elastic Agent, Elasticsearch, and Kibana.

Technically that is correct, but I think the interesting part for users would be the APM Integration, rather than the Elastic Agent. Could we generally talk about the APM Integration, and then in the concrete setup section, and in the more detailled paragraph below introduce the Elastic Agent to run the Integration?

Maybe we could already de-emphasize the difference between spans and transactions a bit. They are written to the same data stream now, so maybe let's focus on traces and then a bit less prominent mention the difference between spans and transactions.

RE: This image:
https://www.elastic.co/guide/en/apm/guide/current/apm-components.html

Another viable scenario will be to host an elastic-agent with an apm integration alongside a user's app, and then have these remote agents enroll via the centrally hosted elastic-agent.