Create an example janitor configuration to cleanup Keptn resources

[RealAnna]

It would be nice to provide users an example on how to automatically clean Keptn resources using Kube-janitor.
The output of this ticket could be a rule file https://codeberg.org/hjacobs/kube-janitor#rules-file to delete jobs, tasks, and workloads.
The base case of Janitor doesn't work because annotations don't work

Acceptance Criteria

• Describe how Janitor can be used to clean up old Keptn CRs and keep it to the bare minimum
• List the Keptn (and created by Keptn) resources that can go out of hand
• Provide an example of Janitor rule
• Link Janitor docs

DoD

Users have an entry point to use Janitor to clean up resources generated by Keptn.