[8.x](backport #4366) Align formatting of APM rules with other rules docs

[mergify]

Description

Aligns formatting of APM rules with other rules docs. This includes:

• Splitting the current APM rules docs from one long page, to a separate page for each rule.
• Adding lists of filters, conditions, action types, and valid action variables.
• Adding an intro to APM rules in the APM section of the Observability guide with links to relevant rules docs.

So far, I've fully written the APM Anomaly rule page. If the team thinks this is a good approach, I'll finish building out the content on the pages for the rest of the APM-related rules.

Preview: https://observability-docs_bk_4366.docs-preview.app.elstc.co/guide/en/observability/master/apm-alerts.html

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 #4040

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)
  • Other:
    I'm not going to port to serverless right now because:
    • In the serverless docs, the APM rules are already in the alerting section.
    • There are some different options for serverless rules vs stateful rules.
    • We can align on the formatting when addressing [Request]: Improve organization and structure of docs about creating/managing rules #4128.
      Let me know if you have any concerns about this approach!
• 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 #4366 done by [Mergify](https://mergify.com).

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