Document velero-policy.json

Right now on http://127.0.0.1:8000/admin/backup/prepare-backups/#preparation we're referring readers to https://github.com/vmware-tanzu/velero-plugin-for-aws?tab=readme-ov-file#option-1-set-permissions-with-an-iam-user but that page uses user `velero` and that's not what we're doing with k0rdent.