Restructure Package Metadata Database documentation and split the offline quick start guide

doc/topics/offline/quick_start_guide.md is a guide about installing GitLab offline, and most of it is the Package Metadata Database download procedures. There is no page anywhere that says what the database actually is.

The guide becomes a high-level overview that links out, and the database gets its own pages: one for what it is, one for how you download it offline. Proposed by @axil in note_3781790330.

Merge requests

They stack in this order, each targeting the one above:

  • !257844 (merged) removes the Change note for the 16.2 vendor/package_metadata_db rename. Nothing below 17 is supported, and both @axil and @nilieskou agreed it can go.
  • !257845 (merged) renames "Using the gsutil tool to download the package metadata exports" to "Download v2 package metadata with gsutil", and fixes the inbound anchor. Requested by @nilieskou in note_3780137769.
  • !253578 (merged) adds doc/user/application_security/detect/package_metadata_database.md: what the database is, what it contains, how sync works, and which data format version each GitLab version reads. The version matrix was asked for in the same note.
  • !257846 (merged) moves the download procedures to doc/topics/offline/package_metadata_database.md, leaves an overview in the guide, and repoints the pages that linked into the moved sections.
  • !257847 (merged) links the new concept page from the offline deployments page. The procedures do not move under offline_deployments/, because that page already routes readers per scanner, per @nilieskou in note_3782514157.

Relates to

Edited by Nick Ilieskou