Add kustomization example for changing to a private registry

Created by: caugustus-sourcegraph

A common use case for kustomize is to change image references to a private registry. We could provide an example, or perhaps an overlay, that demonstrates a maintainable way to run this operation.

For example:

images:
  - name: index.docker.io/sourcegraph/alpine-3.14
    newName: custom-registry-url/sourcegraph/alpine-3.14
    newTag: release-3.40.0