Move the offline package metadata procedures to their own page

What does this MR do and why?

Moves the Package Metadata Database download procedures out of doc/topics/offline/quick_start_guide.md onto a new page, doc/topics/offline/package_metadata_database.md. The guide is about installing GitLab offline, and 822 of its 1211 lines were one feature's procedures. The guide keeps a short section pointing at the new page and at !253578 (merged) for what the database is. Eleven other pages linked into the sections that moved, so their links are updated too.

The new page sits in the same directory as the guide, so every relative link in the moved content still resolves and the move itself is a straight cut and paste with heading levels shifted up one. The data format version table is dropped rather than moved, since !253578 (merged) now holds the version matrix.

Requested by @axil in note_3781790330.

Relates to

• #627684 (closed)
• Stacked on !253578 (merged). !257847 (merged) targets this branch.