docs: Instructions for Magnum image use unclear

In the docs for enabling the containers as a service feature it is unclear that you can (and should) use a plain Ubuntu cloud image as the image for your k8s cluster.

The only section that touches on the images states:
```
Set the following properties on the image that will be used to deploy instances
os-distro to ubuntu
kube-version to workload cluster kubernetes version
openstack image set IMAGE --os-distro ubuntu --property kube_version=v1.32.8
```
but never mentions *which* image to make these changes on.

Since you usually need to build a custom image for magnum as is the case with Fedora images and legacy heat based Ubuntu images, this is quite a difference and should be documented somewhere.

It should be noted that if you use a custom built CAPI Ubuntu image for the CAAS feature the building of your controller VM will fail with the non-de script message of `command k8s could not be found` since the cloud-init seems to expects snaps to either be installed or will install them if the command does not exist in stock Ubuntu cloud images, but not those produced by [image-builder](https://github.com/kubernetes-sigs/image-builder).

Start by locating the documentation section for enabling containers as a service and review its current image instructions. Clarify that a plain Ubuntu cloud image can be used for the Kubernetes cluster, distinguish it from custom Magnum or image-builder images, and document the reported custom-image cloud-init limitation; update the relevant docs and verify the commands and wording.

Written by the indexing model from the issue text.