Document SSH usernames for KubeOne / MachineController

As a current example shows (RHEL on AWS, username `ec2-user`) it's super hard to find out what user for what OS needs to get used at the machine-controller managed workers. As the kubeone manifest has some own ssh user name field, users could think about to use this username. At KKP the user is displayed in the UI, but for kubeone / machine-controller users only it's hard to find out.

As a quick fix we should document on what Cloud / OS what user needs to get taken to connect to a dynamic worker by ssh.
Later we potential should also include the ssh-user name in the status output of the Machine Object, so user could find this information at least there.

If user e.g. changes in regards of custom-images, we should also note this down.

Start by locating the existing RHEL-on-AWS example and review how SSH access details are currently documented for KubeOne and MachineController-managed workers. Document the applicable username by cloud and operating system, including the caveat for custom images; treat adding the username to Machine Object status as separate future work.

Written by the indexing model from the issue text.