Document how to auth to Google Artifact Registry
Document how to auth to Google Artifact Registry

Required now that some of the images we deploy (particularly,
the node placeholder) live on Google Artifact Registry. You
can't deploy a new hub image without this.

berkeley-dsep-infra/datahub,ryanlovett/datahub,berkeley-dsep-infra/datahub,berkeley-dsep-infra/datahub,ryanlovett/datahub,ryanlovett/datahub