[Alerts][Docs] Added API documentation for alerts plugin

[YulNaumenko]

Added missing docs for the alerts API.
Link to test https://kibana_91067.docs-preview.app.elstc.co/guide/en/kibana/master/alerts-api.html
Resolve #49705

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[gmmorris]

It might be worth explicitly stating that the Consumer and Producer fields have to match the Kibana Feature names, as that dictates the required RBAC privileges.
Otherwise people won't know what values to put here I think.

[YulNaumenko]

Yes, it makes sense to me.

[gmmorris]

I think we usually use the term "Alerting framework", rather than "alert framework".
Just making sure we're consistent where we can be :)

[gmmorris]

The actions object is complex enough to warrant documentation I think.
Can we add docs for each of these fields?

{
      "actionTypeId": ".server-log",
      "group": "threshold met",
      "params": {
        "level": "info",
        "message": "alert {{alertName}} is active for group {{context.group}}:\n\n- Value: {{context.value}}\n- Conditions Met: {{context.conditions}} over {{params.timeWindowSize}}{{params.timeWindowUnit}}\n- Timestamp: {{context.date}}"
      },
      "id": "dceeb5d0-6b41-11eb-802b-85b0c1bc8ba2"
    }

[gmmorris]

LGTM, though I've left some questions worth expanding on (such as RBAC consideration of consumer/producer, and the actions field).

[gchaps]

Permanently remove an alert?

[YulNaumenko]

Yes, sounds right for me. Current API doing just a permanent remove with no recovery.

[ymao1]

LGTM! Left some minor formatting comments.

[ymao1]

The Schedule format is also referenced in the Create API doc. Should we link to this from there? Or would it make sense to put Schedule format in the Create doc and link to that from here?

[gchaps]

LGTM

[gchaps]

Suggest putting "Mute single" before "Mute all". Same with Unmute.