Document the PlantUML integration's new owner

Asherah Connorrequested to merge
kiv/document-the-plantuml-integration-s-new-owner into master

What does this MR do and why?

See PlantUML service should be maintained by Planne... (#630440) and PlantUML owned by Planner Intelligence (gitlab-com/runbooks!11638 - merged).

Suggested by the current owners; accepted by the new owner.

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