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/host_vars/prospero.incus.yml

@@ -2,7 +2,6 @@
 # Prospero Configuration - PPLG Observability & Admin Stack
 # Services: pplg (PgAdmin, Prometheus, Loki, Grafana + OAuth2-Proxy)
 
-ansible_user: robert
 
 services:
   - alloy