[kibana] add documentation for alert metricsets

[klacabane]

Summary

Closes #3977

Adds the standard documentation for the new metricsets introduced in elastic/beats#29899, the data streams were created in #3965

[klacabane]

@elastic/response-ops I've added a placeholder[1] description for each metricset, could you please provide a short summary of what these metricsets are ? This documentation is publicly accessible and will show up in the Kibana Integrations UI when looking at the kibana package.

These are the new metricset that need a summary: cluster_actions cluster_rules node_actions node_rules

[1] {metricset} metrics documentation

[elasticmachine]

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• Start Time: 2022-08-09T16:38:24.407+0000

• Duration: 16 min 58 sec

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[miltonhultgren]

I'm a bit confused by how some of the docs are written, for the ECS fields we seem to be documenting what the ECS field means but also how it should be populated when implementing the spec, but doesn't isn't the integration the implementation already?

[klacabane]

@miltonhultgren so this is what the type: external is for, to pull the documentation from the ecs repo (see service fields https://github.com/elastic/ecs/blob/main/schemas/service.yml#L176). It definitely is confusing, we should document each field with the specifics of the integration and not the generic description.

We can pass a description property to the fields that will override the default ecs description output to the README.

[klacabane]

No updates from @elastic/response-ops, I've also notified the team in slack. Let's merge as-is and follow up with a separate PR