[8.14](backport #4218) [apm] Add headings to APM known issues page

[mergify]

Closes #4212

Adds headings to the APM known issues page to make it easier to read. The heading text is longer than we prefer, but I think it's ok for this page only (@elastic/obs-docs let me know if you disagree).

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This is an automatic backport of pull request #4218 done by Mergify.

[mergify]

Cherry-pick of fc7581c has failed:

On branch mergify/bp/8.14/pr-4218
Your branch is up to date with 'origin/8.14'.

You are currently cherry-picking commit fc7581cf.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docs/en/observability/apm/known-issues.asciidoc

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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