Etcd backup and restore for management clusters

**What would you like to be added**:
Review the etcd backup and restore docs and make sure we give instructions for how to restore management clusters that are already managing workload clusters.

Start by locating the etcd backup and restore documentation and reviewing its existing management-cluster guidance. Trace how the documentation addresses clusters that already manage workload clusters, then update it with the missing restoration instructions. Done means a reader can follow the documented process for this scenario.

Written by the indexing model from the issue text.