docs: mark registry.k8s.io as official and GCR/quay as secondaries

[AkashKumar7902]

Refs #20928

What changed

• Make GitHub Releases the default binary download source in generated Linux and macOS release notes while retaining Google Storage as an alternate.
• Document registry.k8s.io/etcd as the official multi-architecture image location and GCR/Quay as secondary registries.
• Update the runnable Docker example and container name to use the official image.

Why

The release-note template still directs users to Google Storage by default and describes GCR as the primary container registry. That no longer matches the staged distribution policy agreed in #20928 or the currently published release artifacts.

Impact

Future release pages direct users to the official binary and container locations without removing the existing compatibility sources. This documentation-only change does not alter the release pipeline or complete the broader deprecation work tracked by #20928.

Testing

• git diff --check
• Rendered scripts/release_notes.tpl.txt with the same substitutions as scripts/release.sh, verified that all release placeholders resolved, and syntax-checked every generated sh block with bash -n.
• make verify-shellcheck
• make verify-shellws
• make verify-markdown-marker
• Verified representative GitHub Releases and Google Storage Linux/macOS asset URLs with curl -fsSIL.
• Inspected registry.k8s.io, GCR, and Quay manifests for v3.7.0; the official manifest includes amd64, arm64, ppc64le, and s390x.

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: AkashKumar7902
Once this PR has been reviewed and has the lgtm label, please assign ivanvc for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• scripts/OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[kubernetes-prow]

Hi @AkashKumar7902. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.