Link the Package Metadata Database page from offline deployments

What does this MR do and why?

Links doc/user/application_security/detect/package_metadata_database.md from the Package Metadata Database section of doc/user/application_security/offline_deployments/_index.md, so a reader there can find out what the database holds and which data format version their GitLab version reads before they follow the setup link.

The setup instructions themselves stay where they are. @axil asked in note_3781790330 why they are not on the offline deployments page, and @nilieskou answered in note_3782514157 that the page already routes readers per scanner, so moving them would duplicate that routing.

Relates to

• #627684 (closed)
• Last in the stack, on !257846 (merged).