[doc] - Add how to release new versions of the kube-rbac-proxy image used

[camilamacedo86]

What do you want to happen?

The projects built with KB create a side container with kube-rbac-proxy
The kube-rbac-proxy protects the manager/operator.

However, we need to have the images built in: https://console.cloud.google.com/gcr/images/kubebuilder/GLOBAL/kube-rbac-proxy so that we do not risk losing them and breaking the projects.

The project kube-rbac-proxy is in the process to be donated to the k8s org. However, it is going on for a long time and we have no ETA for that to occur. When that occurs we can automate this process. But until there we need to generate these images.

To generate the images we need to bump the version on the branch kube-rbac-proxy-releases see #2578 then the script will generate it.

See that the branch has a README that explains the steps. However, unless you look for the branch you have no idea how these images are built currently. So the idea of this task is we add this info and link the branch README to the Contribution Guide: https://github.com/kubernetes-sigs/kubebuilder/blob/master/CONTRIBUTING.md

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[camilamacedo86]

It was discussed in the Kubebuider meeting on April 07 and accepted by the community.