[WIP]Modifying registered clusters documentation

[divya-mohan0209]

Fixes rancher/rancher-docs#77

Tasks to be accomplished per the conversation in the aforementioned issue for v2.5 & v2.6 docs

• [x] Clarify that importing still exists along with registration
• [x] Differentiate between admin & dev workloads on a k8s cluster
• [ ] After point. 2 is completed, differentiate based on how cluster administration is managed

cc: @deniseschannon @cbron