doc: Use case examples for PR-CI checker tool
    
    This document showcases common usecases for the user to
    interact with the PR-CI checker tool.
    
    Signed-off-by: David Pascual <davherna@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>