[Monitoring][Doc] Update out of the box alerting page

[igoristic]

We need to include/update the new out of the box alerts in this page that were included in 7.10:

• CPU Usage
• Memory Usage
• Disk Usage
• Missing Monitoring Data
• Thread Pool Rejections
• CCR Read Exceptions
• Large Shards

Maybe also docs for:

• The six default cluster alerts (which will be migrated to Kibana Alerts after [Monitoring] Migrate data source for legacy alerts to monitoring data directly #87377 is merged)
• Might need to change "Missing Monitoring Data" alert that it's only ES product for now (to avoid any confusion)

[elasticmachine]

Pinging @elastic/stack-monitoring (Team:Monitoring)

[igoristic]

cc: @lcawl

[chrisronline]

Going to reopen this.

We need docs for the threadpool alerts too.

[igoristic]

Opening this issue since we still need to do:

Might need to change "Missing Monitoring Data" alert that it's only ES product for now (to avoid any confusion)

As part of the acceptance criteria defined in this ticket

[ravikesarwani]

Thanks @igoristic for taking care of making the help better for the Missing Monitoring Data.

Few other things that I see we should also change in the same help page, that seems incorrect right now:

• This line should be deleted because we have migrated all the alerts to kibana alerts "The alerts are visible alongside your existing Watcher cluster alerts."

• Change the following line "To review and modify all the available alerts, use Alerts and Actions in Stack Management." to "To review and modify all the available alerts, use Enter setup mode on the Cluster overview page in Stack monitoring.

• We should add the following lines to handle Watcher to Kibana alert migration as a “Note”. This Note can be added just after the previous line “To review and modify …”.
  “Default Watcher based alerts for stack monitoring has been recreated as kibana alerts. This causes the existing Watcher email actions to not be generated any more. The default actions for these out-of-the box kibana alerts are Write to Kibana logs. Review and modify all the available stack monitoring alerts using the Enter setup mode on the Cluster overview page and add additional Email action to continue receiving email alerts.”

• "CPU threshold" as the alert name should be changed to "CPU usage threshold" to be consistent with Disk and JVM

• Can you confirm if the CPU, Disk and Memory usage threshold applies only to Elasticsearch nodes? If that's the case we should modify the help for these alerts also to mention that its for Elasticsearch nodes only. Example "This alert is triggered when a Elasticsearch node runs a consistently high CPU load..."