Document capm3 vs capo workload cluster os_images update workflow

Summary

Add some doc in docs/sylva-os-images.md around os_images (see their use in https://gitlab.com/sylva-projects/sylva-elements/helm-charts/sylva-capi-cluster/-/blob/0.2.20/README.md?ref_type=tags) generation for an workload cluster image update (

• for CAPO cluster, unit in workload cluster creates it from workload cluster values - ./apply-workload-cluster.sh environment-values/workload-clusters/rke2-capo, to be used also for get-openstack-images pushing to Glance,
• while CAPM3, an upgrade of the management cluster is needed - ./apply.sh environment-values/rke2-capo).
  

There's no os-image-info unit in deployed for a CAPM3 workload cluster due to the fact that management cluster's unit shared-workload-clusters-settings is handling the cloning of ConfigMap/os-images-info -n sylva-system into ConfigMap/kyverno-cloned-os-images-info-capm3 -n <workload cluster ns> as soon as the workload cluster ns is created.

related references

Details