Document how to use External CI Jobs for Jenkins Pipelines

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Proposal

In #421913 we can use "External CI Jobs" via &10866 to solve that use case:

You would have an external job on a GitLab pipeline that would be updated with the status of Jenkins pipeline. This way both external and internal pipelines are merged and you would have both MR reports coming from the GitLab pipeline as well as visibility into the external processing.