docs(byoc): add GitOps cluster registration

[sbaum1994]

TL;DR

Document an external-organization BYOC registration workflow that uses a SAK
for the one-time NGC API bootstrap and GitOps for the NVCA operator lifecycle.
The guide is needed because the existing managed-cluster pages focus on UI
registration and do not show the no-Vault, no-SSA path.

Additional Details

• Adds an idempotent ngc cf cluster bootstrap suitable for CI.
• Defines the boundary between non-secret Helm values and GitOps-managed
  Kubernetes and chart repository credentials.
• Provides the Helm values required for a SAK-backed, Helm-managed deployment.
• Calls out that the public NGC CLI cannot currently set the UI management mode
  to read-only.
• Links the new guide from the Helm-managed page and Fern navigation.

For the Reviewer

Review the commands and chart values in
docs/ngc-managed/cluster-management/gitops-registration.md, especially the
credential handling and management-mode limitation.

For QA

QA is not required for this documentation-only change. The author will test the
workflow against an external organization before the PR is marked ready.

Validation completed:

• ./tools/ci/check-docs (0 errors; the existing version-sync check remained
  advisory because it starts without imports.yaml)
• git diff --check
• Rendered the documented secret-backed values with the current approved NVCA
  operator chart during authoring

Issues

Closes #1556

Checklist

• I am familiar with the Contributing Guidelines.
• I have signed off my commits for Developer Certificate of Origin (DCO) compliance.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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