This is a followup of RHCLOUD-27610

The Slack notifications must be handled at Jenkinsfile level, as "publishers" don't work in JJB job template definitions for pipeline types.

We need to add a snippet in the post section of the pipeline to send a notification using the Slack Jenkins plugin or similiar approach in declarative pipelines (those which use a Jenkinsfile).

We need to update the documentation to reflect how to configure this.