It wasn't clear how to configure Google Application Default
credentials with the registry, so this commit adds some comments and
updates the documentation link.

The previous link to
https://docs.docker.com/registry/storage-drivers/gcs no longer
exists. The CNCF docs in
https://distribution.github.io/distribution/storage-drivers/gcs/ are
correct today, but it's possible that the CNCF docs adds or changes
parameters that are not supported by the GitLab Container Registry.
Still, the documentation there is better than nothing.

In the future, we may want to sync storage driver changes with the
CNCF project
(https://gitlab.com/gitlab-org/container-registry/-/issues/1240).

Relates to https://gitlab.com/gitlab-org/charts/gitlab/-/issues/3434