Document the self-managed 0.6.1 to 0.14.4 upgrade

Report needed documentation

Self-managed NVCF customers upgrading from 0.6.1 need a validated procedure
for reaching 0.14.4. This release range crosses Cassandra and OpenBao
migrations, an NVCT schema compatibility boundary, and control-plane and
compute-plane chart changes.

Describe the documentation

Add a customer upgrade guide that covers:

• Prerequisites, backups, configuration reconciliation, and immutable pins.
• Selector-scoped Cassandra and OpenBao migration checkpoints.
• A targeted NVCT hard stop while the NVCF API and function path stay online.
• Migration ledger, NATS stream, service health, and rollback checks.
• Compute-plane registration refresh and NVCA upgrade steps.
• Function, deployment, database row, and worker continuity validation.

Verification completed

• Compared the 0.6.1 and 0.14.4 stack artifacts and database migrations.
• Inspected service runtimes for scheduled migration hard stops.
• Exercised the upgrade on a fresh local k3d single-cluster installation.
• Preserved the same function, deployment, database rows, worker pod, and
  container identities through the upgrade.
• Ran the repository documentation checks.

Steps taken to search for existing documentation

Reviewed the current release-notes index, existing upgrade guidance, stack
manifests, and self-managed installation documentation. No direct 0.6.1 to
0.14.4 procedure exists.

Start with the release-notes index, existing upgrade guidance, stack manifests, self-managed installation documentation, and the 0.6.1 and 0.14.4 migration artifacts. Trace the Cassandra, OpenBao, NVCT, control-plane, and compute-plane checkpoints, then document the validated procedure and run the repository documentation checks. Done means the guide covers the listed prerequisites, migration, rollback, continuity, and verification steps.