Document cover images

Many knowledge-base tools let you add a cover or banner image to a page, which makes spaces feel more distinct. This was previously requested for collections in #2478 (auto-closed as stale). I'd like to propose it for documents, and I'm willing to implement it.

Proposed behaviour
• Editors can add a cover below the title and meta line. It's full width with rounded corners. They can upload it or drag an image in.
• The cover can be changed, repositioned vertically by dragging, or removed.
• The cover shows on public shares.

Proposed approach
• Add coverUrl and coverPosition columns on documents and expose them through documents.update.
• Add a new DocumentCover attachment preset with a public-read ACL (like avatars) so covers load on shares. SVG uploads are rejected.
• Covers are copied when a document is duplicated.