[blocked] [apm] Add info on critical path and aggregated critical path

[colleenmcginnis]

Description

Adds information about the experimental critical path view and aggregated critical path view.

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 #2876

Checklist

• Add information about critical path
• Add information about aggregated critical path
  ⚠️ Blocked: Aggregated critical path view is not currently working as expected in the latest Elastic Stack release. The dev team is aware (see [REQUEST]: Critical path and aggregated critical path #2876 (comment)). I'm waiting to add info on the aggregated critical path view until it's available again.
• 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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[mergify]

This pull request is now in conflict. Could you fix it @colleenmcginnis? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b issue-2876 upstream/issue-2876
git merge upstream/main
git push upstream issue-2876