docs: document terraform import for incus containers and remove korax

Add comprehensive terraform import documentation for Incus resources.
Includes syntax for importing containers with for_each keys, retrieving
image fingerprints, and specific import commands for Uranian hosts.
Covers troubleshooting for common import issues and state verification.
Removes obsolete korax.helu.ca host from ansible inventory.

ansible/inventory/hosts

@@ -17,7 +17,6 @@ ubuntu:
     rosalind.incus:
     sycorax.incus:
     titania.incus:
-    korax.helu.ca:
 
 # Service-specific groups for targeted deployments
 agent_s: