Correct tenant-setup.md: VN/Subnet required for network-attached compute

[tchughesiv]

Summary

docs/guides/developer/tenant-setup.md contains a stale note (lines ~422–425) stating VirtualNetwork and Subnet are not required for ComputeInstance creation. That is incorrect for CUDN / network-attached compute instances, which OSAC-244 validation exercised.

Evidence (current codebase)

• Public API uses spec.network_attachments; attachments must reference a READY Subnet
• Operator places VMs in the subnet target namespace when networkAttachments is set (osac-operator/internal/controller/computeinstance_controller.go)
• Stale note says network attachment is not yet required — attachment is already part of the API/operator model

Nuance (keep doc accurate)

VN + Subnet are required when using:

• CUDN-backed network classes, or
• Any compute instance created with --network-attachment "subnet=${SUBNET_ID}"

They are not required for the legacy/minimal path: ComputeInstance CR with no networkAttachments (VM in tenant namespace only). Document both paths briefly.

Requested changes

1. Remove or replace the note at lines ~422–425 (“not yet required … will be updated when network attachment becomes a required step”).
2. Add a short “Network-attached compute instances” subsection: VN + Subnet READY before create; link to updated computeinstance-guide.md.
3. Add cross-reference on the ComputeInstance CR example (~448–467) that it omits networkAttachments (tenant-namespace path only).

References

• OSAC-244 validation