Set up a GCS bucket for serving `gitlab-docs` Review Apps

Similarly to https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/8203#note_243150421, we'd like to host documentations Review Apps in a GCS bucket served through Cloudflare.

The setup should almost the same as for www-gitlab-com Review Apps.

• The domain would be *.docs.gitlab-review.app and managed in https://ops.gitlab.net/gitlab-com/gitlab-com-infrastructure/-/blob/master/environments/dns/gitlab_review_app.auto.tfvars.json#L59-66.
• The GCS bucket should probably be under Marketing > Documentation (similarly to https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/8203#note_246351579).
• The CI config for https://gitlab.com/gitlab-org/gitlab-docs should be updated similarly to gitlab-com/www-gitlab-com!35869 (diffs).

Related to gitlab-org/gitlab-docs#735 (closed).

/cc @kwiebers @axil

/cc @alejandro since you worked on https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/8203 and gitlab-com/www-gitlab-com!35869 (merged).