Document sync of advisories for self-managed instances

Expand on https://docs.gitlab.com/ee/topics/offline/quick_start_guide.html#enabling-the-package-metadata-database and document how to sync the advisories. The GCP bucket is a different one.

Implementation plan

• add section for how to add and sync advisories dir
• mention old licenses dir and new one (package_metadata vs package_metadata_db)