[8.x](backport #4714) [apm] Clarify requirements in the Fleet-managed APM Server guide

[mergify]

Description

Updates the Fleet-managed APM Server guide to address feedback from @eedugon including:

• Let users know that they don't have to set up a Fleet Server if they already have one set up.
• Let users know that that the APM integration doesn't have to run in the same agent that acts as the Fleet Server, but that is the easiest way to get started.

There are some points that @eedugon brought up in #4668 that are not addressed here including:

• Shouldn't we rely and refer to the Fleet documentation for the Fleet Setup (if it's not already done)?
  The content for both the Prerequisites and Step 1: Set up Fleet sections come from the Fleet docs (here and here). We could link out to the Fleet guide instead of importing this content into this APM doc, but in my opinion we should only do that if it's the best user experience, not because of maintenance concerns. However, it would probably be a good idea to review the content in the elastic/ingest-docs repo to make sure it's up to date and as clear as possible.
• Regarding the diagram on the getting started overview page: I don't think we should over complicate this diagram. The goal of this diagram is to help a new user get a high-level understanding of how the pieces relate to each other. If we're adding clarification in the step-by-step guide, I think it's ok if this diagram shows the simplest implementation.

@eedugon @bmorelli25 thoughts on this approach?

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Closes #4668

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

---

This is an automatic backport of pull request #4714 done by [Mergify](https://mergify.com).

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