What's the task? Please describe

We are often asked about specific tips for node maintenance during the K8s upgrade when users use Rancher to manage their clusters.

The goal is to update the current doc https://longhorn.io/docs/1.4.0/volumes-and-nodes/maintenance/#updating-kubernetes, to include how to cordon & drain nodes the downstream nodes of the cluster via Rancher to ensure live volumes w/o disruption.

We will extend this doc for other management solutions if need or any request from the community in the future as well.

Describe the items of the task (DoD, definition of done) you'd like

• Improve https://longhorn.io/docs/1.4.0/volumes-and-nodes/maintenance/#updating-kubernetes

Additional context

Need QA to verify this doc as well.

cc @longhorn/dev