Merge pull request #57473 from spiffxp/add-code-of-conduct-to-staging…

…-repos

Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Add code-of-conduct.md to staging repos

Instead of adding it directly to their published copies

Replaces
- #13
- kubernetes/apiextensions-apiserver#21
- kubernetes/apimachinery#33
- kubernetes/apiserver#28
- kubernetes/client-go#350
- kubernetes/code-generator#29
- kubernetes/kube-aggregator#11
- kubernetes/metrics#12
- kubernetes/sample-apiserver#18
- kubernetes/sample-controller#3

ref: kubernetes/community#1527

Kubernetes-commit: 8a4a39d6ae1be45115c12cfa166f2c8151d88d8c