docs(node pools): checks for nodes in node pools changing role

[PaulRMellor]

Documentation

Adds a section on changing the roles of a node pool and the checks made when a broker role is removed.
Also removes out-of-date warnings on using KRaft mode.

Checklist

Please go through this checklist and make sure all applicable tasks have been done

• Write tests
• Make sure all tests pass
• Update documentation
• Check RBAC rights for Kubernetes / OpenShift roles
• Try your changes from Pod inside your Kubernetes and OpenShift cluster, not just locally
• Reference relevant issue(s) and close them after merging
• Update CHANGELOG.md
• Supply screenshots for visual changes, such as Grafana dashboards