[8.13](backport #3676) Add rate and percentile aggregations to docs for custom threshold rule

[mergify]

Adds changes requested in https://github.com/elastic/observability-docs/issues/3530.

TODO:

• Add rate and percentile aggregations.
• Create a dedicated section/page for rate aggregation that:
  • Explains why you would use rate instead of average for something like a counter.
  • Shows how the rate is calculated.
  • Provides a realistic example.
    Then point to the page from both rules.
• Refine the new content about rate aggregation and add example (with input from development team)
• Port this change to the serverless docs, too (making sure to add the missing header).

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This is an automatic backport of pull request #3676 done by [Mergify](https://mergify.com).

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