feat: add Quay.io as additional OCI registry target for chart publishing

[hamdikh]

Summary

Charts are currently published to GHCR as OCI artifacts. Adding Quay.io as a second OCI target increases availability and mirrors what other CNCF projects do to reduce single-registry dependency.

What needs to be done

• Create a Robot Account under the thanos org on Quay.io
• Store credentials as repository secrets: QUAY_USERNAME, QUAY_PASSWORD
• Extend .github/workflows/release.yml to push to Quay.io after GHCR:

- name: Push chart to Quay.io OCI
  run: |
    helm registry login quay.io \
      --username ${{ secrets.QUAY_USERNAME }} \
      --password ${{ secrets.QUAY_PASSWORD }}
    helm push .cr-release-packages/*.tgz oci://quay.io/thanos-community/helm-charts

• Update README.md and ArtifactHub metadata to reference both registries
• Add Cosign signing step for the Quay.io push (mirrors GHCR flow)

References

• https://docs.quay.io/guides/create-repo.html
• https://helm.sh/docs/topics/registries/

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