Add LLM Instance Gateway to staging project list

[kfswain]

Our project: https://github.com/kubernetes-sigs/llm-instance-gateway has need to create images in a public repo.

Followed: https://github.com/kubernetes/k8s.io/blob/main/registry.k8s.io/README.md#creating-staging-repos to cut this PR

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[kfswain]

/retest

[kfswain]

@dims @upodroid Heya folks! Anything you need from me to get this rollin? AFAIK this PR is needed to be able to push test containers to a k8s registry. Thanks!

[ahg-g]

/lgtm