Document the GPU Operator Helm chart OCI artifact

[mikemckiernan]

Summary

The GPU Operator Helm chart is now publicly available as an OCI artifact in
NGC. Document this as a v26.7.0 feature and add OCI installation instructions.

Because the artifact became available after the initial v26.7.0 release,
include a brief post-release note explaining the timing.

Changes

New Features

Add an entry under 26.7.0 > New Features:

* Added the GPU Operator Helm chart as an OCI artifact in NGC.
  You can now install the Operator directly from
  ``oci://nvcr.io/nvidia/cloud-native-charts/gpu-operator`` as an alternative
  to using the classic Helm repository.
  (`Issue #2520 <https://github.com/NVIDIA/gpu-operator/issues/2520>`__)

Installation guide

Update gpu-operator/getting-started.rst with an OCI installation alternative.
Retain the existing classic Helm repository instructions.

Example:

$ helm install --wait gpu-operator \
    -n gpu-operator --create-namespace \
    oci://nvcr.io/nvidia/cloud-native-charts/gpu-operator \
    --version=${version}

Post-release note

Add this one-line entry under 26.7.0 > Post-Release Documentation Updates:

* Added the Helm chart OCI artifact to the new features and installation
  documentation after the artifact became available following the v26.7.0 release.

Acceptance criteria

• The OCI artifact is described under v26.7.0 New Features.
• The installation guide includes a validated OCI installation command.
• The classic Helm repository workflow remains documented.
• The post-release entry explains why the feature was not documented in the
  original v26.7.0 release notes.

References

• [Feature]: Publish the gpu-operator Helm chart as an OCI artifact gpu-operator#2520
• https://catalog.ngc.nvidia.com/orgs/nvidia/cloud-native-charts/containers/gpu-operator/v26.7.0