docs: added additional GCS authentication information (backport release-3.2.x)

[loki-gh-app]

Backport 39c47e5 from #15112

---

What this PR does / why we need it:

Added some basic info about GCS authentication.

Special notes for your reviewer:

Not all of Loki/GEL's services support ADC. I had to use loki.structuredConfig.admin_client.storage.gcs.service_account to force the Admin API to use service account credentials. It did not seem capable of utilizing GOOGLE_APPLICATION_CREDENTIALS.

I'm uncertain how to document this other than to exhaustive documentation. Maybe such detail is more appropriate in the Helm chart docs/examples?

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR