📖 Update Resources Design Document to use PlantUML

[christopherhein]

Vince had pointed us to that CAPI providers typically have a make target to help create diagrams that are managed in the repo, this issue is to go back and make the control plane resources using PlantUML https://github.com/kubernetes-sigs/cluster-api-provider-nested/issues/7#issuecomment-735514947

[fejta-bot]

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[vincepri]

/lifecycle frozen