Add a Package Metadata Database page

What does this MR do and why?

Adds doc/user/application_security/detect/package_metadata_database.md, a single page for what the Package Metadata Database is: the datasets it holds, the two data format versions, which version each GitLab version reads, and how synchronization works online and offline. Until now the only description of the database lived inside the offline quick start guide, mixed into the download procedures.

The page is conceptual. The procedures stay in the quick start guide and move to their own page in !257846 (merged).

Requested by @axil in note_3781790330 and restated as the biggest win in note_3880253940. The data format version matrix was requested by @nilieskou in note_3780137769.

Relates to

• #627684 (closed)
• #611168 (closed)
• Stacked on !257845 (merged). !257846 (merged) targets this branch.