[8.x](backport #4750) [synthetics] Update TLS certificate rule doc to include Synthetics

[mergify]

Description

Updates the TLS certificate rule doc to include both the Uptime TLS rule and the Synthetics TLS certificate rule, and updates the format to match the Monitor status rule doc.

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
  • Note: I'll copy over just the Synthetics section to Serverless after review.
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Closes #4745

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 #4750 done by [Mergify](https://mergify.com).

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