documentation: deploy a preview of the user guide for MRs

Conversation from the documentation channel: https://chat.civicrm.org/civicrm/pl/cn8tx5dnuine8d1xdezunnucra

## Load credentials on a CLI
gcloud auth activate-service-account --key-file=lab-c-o-creds.json

## Use credential to read data
gsutil ls gs://civicrm-ephemera  ## Works properly
gsutil ls gs://civicrm-private   ## Fails properly

## Use credential to write data
mkdir example-3
cd example-3
vi index.html
gsutil rsync -r . gs://civicrm-ephemera/docs/example-3
curl https://ephemera.civicrm.org/docs/example-3/

It should work with this: https://about.gitlab.com/blog/2023/08/21/how-to-secure-cloud-run-deployment-with-auto-devops/

Long-term, this doc might have better advice: https://medium.com/google-cloud/secure-your-gitlab-ci-cd-pipelines-moving-away-from-service-account-keys-e949e59cf065