[8.x](backport #4599) Clean up APM getting started intro page

[mergify]

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>

---

This is an automatic backport of pull request #4599 done by [Mergify](https://mergify.com).

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