Document the v3 offline vendor layout for licenses

What does this MR do and why?

Documents the v3 offline vendor layout for licenses in the offline quick start guide, which had no v3 layout: the registry-id directory names (cross-linked to the malware advisory mapping, gem data lives under v3/rubygem/, not v3/gem/), the full_dataset snapshot structure, and the mirror growth (about a quarter, roughly 0.5 GiB as of GitLab 19.4).

The doc describes the merged end state: in GitLab 19.4 and later, license synchronization reads the v3 layout, per the #622511 (closed) cutover (plan and diagram: LicenseSyncWorker flips to the v3 sync service under #606527 (closed)). Merge order is enforced with MR dependencies on !250674 (merged) (v3 ingestion) and !251527 (merged) (PDS switch); when the #606527 (closed) cutover MR opens, it replaces them as the blocker. Troubleshooting guidance for the layout warnings ships with the MRs that ship the warnings.

Blocked by

• !250674 (merged)
• !251527 (merged)

References

Relates to #611168.