Clean up APM getting started intro page

[colleenmcginnis]

Description

Cleans up the APM getting started intro page as discussed in #4336 including:

• Which option is the "easiest": @simitt @bmorelli25 what do you think about just removing the note at the top and letting users determine which is the easiest method for their situation? If we can't say that one is easier than the other without explaining the context, then I'm not sure it makes sense to have a note at the top.
• What's preventing feature parity: I moved this note to the part of the table that discusses supported outputs.
• Required components: I rephrased the Elastic Agent list item (I was going to use a footnote but I was having trouble using it inside a list that's inside a table).
• I also deleted a big block of commented out content since we can get it from the git history if needed, but let me know if you think we should keep it.

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 #4336

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>

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[bmorelli25]

I'm good with these changes. The Get Started page is now a quick overview, a brief pro/con list of each option, and the flowchart to help people decide. I think there's enough context on the page for users to decide which option is easiest for their use case.

[colleenmcginnis]

@eedugon do you mind if we move your comments to a separate issue? I think updating the diagram and the other subpages in the getting started section are out of scope for this PR, which aims to address the issues listed in #4336.

[eedugon]

@colleenmcginnis , totally! sorry for having jumped in without a clear scope of the changes! 🙏

I'll create an issue for the APM installation guide and its relation with the Fleet Setup, which I think it's probably the most important part of my comment.

[colleenmcginnis]

Moved @eedugon's comments to #4668 and #4669. Hiding them in this PR.