[Fleet]: Document specific agent status alerts

[nimarezainia]

As part of https://github.com/elastic/ingest-dev/issues/6319 we will have out of the box alerting on specific agent status changes. Once completed these status alerts need to be documented.

The new docs should probably go here: https://www.elastic.co/guide/en/fleet/8.19/agent-health-status.html#agent-health-status-outage

As part of this effort could we please also remove this section:https://www.elastic.co/guide/en/fleet/8.19/monitor-elastic-agent.html#fleet-alerting which is no longer superseded and replaced by the much more functional status alerts.

cc @karenzone