charts: publish helm chart on Artifact Hub

[qinqon]

Is this a BUG FIX or a FEATURE?:

/kind enhancement

What this PR does / why we need it:

The chart is published to quay.io/nmstate/kubernetes-nmstate since v0.87.0, but nothing tells users about it and the chart is not discoverable. This adds the missing pieces:

1. Helm instructions in the release notes (hack/release-notes.tmpl). The generated notes only documented the manifest based installation. Rendered output:

   ## With Helm
   
   helm install nmstate oci://quay.io/nmstate/kubernetes-nmstate \
     --version 0.87.0 \
     --namespace nmstate \
     --create-namespace
   

   The helm path does not need the separate NMState CR step, since the chart creates it by default (nmstate.enabled=true).

2. A chart README (charts/kubernetes-nmstate/README.md). Artifact Hub renders the chart README as the package page body, and the chart did not ship one, so the page would be empty. It covers install, values, usage, upgrade and uninstall.

3. Artifact Hub repository metadata (hack/artifacthub-repo.yml), carrying the ownership information.

Special notes for your reviewer:

Listing the chart on Artifact Hub is a manual step that still has to be done: sign in at artifacthub.io, create an nmstate organization and add a repository of kind Helm charts with the url oci://quay.io/nmstate/kubernetes-nmstate. From then on new chart versions are indexed automatically. This PR only provides what the repository side has to contribute.

Two consequences worth knowing:

• repositoryID in hack/artifacthub-repo.yml is empty on purpose, the value only exists once the repository has been added. Filling it in later enables the Verified publisher flag. The owners section alone is enough for the ownership claim.
• The README will only show up on Artifact Hub from the next release on, since Artifact Hub reads it from the chart package and 0.87.0 is already published without it.

The metadata file is pushed by hand with oras (the command is in the file header). It changes rarely, so no release automation was added for it, and the chart push itself is unchanged.

For reference, MetalLB is set up the same way: registered manually, artifacthub-repo.yml with repositoryID plus owners, chart README, and no artifacthub.io/* annotations, which is why none were added here.

Verified with make lint-helm and make whitespace-check, and by packaging the chart to confirm README.md ends up inside the tarball. The rendered release notes were checked against the v0.87.0 ones produced by CI: same changelog body, only the Installation section differs.

Release note:

NONE

[kubevirt-prow]

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[Copilot]

Pull request overview

Adds Helm installation guidance to generated release notes.

Changes:

• Documents OCI-based Helm installation.
• Explains default NMState resource creation.
• Retains manifest installation as an alternative.

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[kubevirt-prow]

@qinqon: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-kubernetes-nmstate-e2e-upgrade-k8s	e19ab47	link	false	/test pull-kubernetes-nmstate-e2e-upgrade-k8s

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