[8.x](backport #4713) [apm] Update APM decision tree diagram

[mergify]

Description

Updates the APM decision tree to address feedback from @eedugon including:

• Clarifies that when we ask about using other integrations, we mean other integrations with APM Server.
• Clarifies what we mean by running APM Server on the edge vs centrally.
  • Adds a brief description to the diagram itself.
  • Adds to the list of pros of using a Fleet-managed APM Server that you can centrally manage multiple APM Servers running on edge machines.

Note: In #4669 @eedugon mentioned that where the user is running APM Server (edge machine vs centrally) doesn't technically determine whether they must use Fleet-managed or APM Server binary. However, the goal of this section is to help a new user make a decision based on several factors not to be a complete reference of all implementations that are or aren't possible with Elastic APM. Is there anything we can change about the intro to the "Help me decide" section to make that clearer? @bmorelli25 any thoughts here?

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 #4669

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 #4713 done by [Mergify](https://mergify.com).

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