Document how to consume post-submit / nightly images

After the recent changes in #2051 we are now publishing images on every merge to the main and release branches.

Additionally for the main branch we tag images with a nightly tag and publish corresponding manifests (e.g. infrastructure-components.yaml & cluster-templates) to GCS.

Let's document where those images are available / how to consume them (e.g. via clusterctl if that is easily possible (?)).

Images:

• Build via: https://prow.k8s.io/?repo=kubernetes-sigs%2Fcluster-api-provider-vsphere&job=post-cluster-api-provider-vsphere-push-images
  • gcr.io/k8s-staging-capi-vsphere/cluster-api-vsphere-controller:main
  • gcr.io/k8s-staging-capi-vsphere/cluster-api-vsphere-controller:v20230802-v1.8.0-beta.1-8-g2fae0a97
  • gcr.io/k8s-staging-capi-vsphere/cluster-api-vsphere-controller:release-1.7
  • gcr.io/k8s-staging-capi-vsphere/cluster-api-vsphere-controller:v20230802-v1.7.1-25-g3ec8d66d
• Build via: https://prow.k8s.io/?job=cluster-api-provider-vsphere-push-images-nightly
  • gcr.io/k8s-staging-capi-vsphere/cluster-api-vsphere-controller:nightly_main_20230802

Manifests:

• Build via: https://prow.k8s.io/?job=cluster-api-provider-vsphere-push-images-nightly
  • Google Cloud UI: https://console.cloud.google.com/storage/browser/artifacts.k8s-staging-capi-vsphere.appspot.com/components/nightly_main_20230802
  • Example for public file URL: https://storage.googleapis.com/artifacts.k8s-staging-capi-vsphere.appspot.com/components/nightly_main_20230802/infrastructure-components.yaml