Documentation for log-based alerting

[Rotfuks]

Motvation

Now that log-based alerts are possible we also need to document how customers can setup alerts based on logs, so they are empowered to do it in self-service. 

Todo

• Extend the public documentation for alerting in https://docs.giantswarm.io/tutorials/observability/ on how to create log-based alert rules.
  • If there is no public documentation for it yet, please add it to the local documentation like https://github.com/giantswarm/giantswarm-management-clusters/tree/main/management-clusters/grizzly/extras/observability and check with the team if this should be finally added to the public one :)

Outcome

• Customers can read up on their own how to create log-based alert rules.