[Docs] Add gcloud auth login in GCP setup instruction

[yika-luo]

Without user running gcloud auth login, they cannot run sky jobs launch as sky will use gcloud CLI such as gsutil in the code.

[Michaelvll]

Seems gcloud init is a superset of gcloud auth login. May need to double check if we really need gcloud auth login. https://cloud.google.com/sdk/docs/authorizing#user-account

[yika-luo]

Seems gcloud init is a superset of gcloud auth login. May need to double check if we really need gcloud auth login. https://cloud.google.com/sdk/docs/authorizing#user-account

gcloud init uses a web-based authorization flow so it doesn't seem to generate any credentials locally, which caused the issue of sky not able to use gsutil during sky jobs launch (#4047)