Support for Header images

Currently, Outline documents are text-centric. While this is great for focus, it lacks the visual branding and "landing page" feel required for high-level project hubs and documents shared externally. I propose adding support for Header Images (Cover Photos) at the top of documents, with a native integration for Unsplash (or similar) to make discovery seamless. Implement a "Add Cover" feature at the top of every document (similar to the current "Add Icon" functionality). This feature should also have an option to reposition the image withing the header frame. Use Cases • Public-Facing Documentation: Companies using Outline for public handbooks can give their pages a branded, professional look. • Project Hubs: Distinguishing a "Project Alpha" home page from its sub-pages using a consistent visual theme. • Onboarding: Using welcoming imagery for new hire "Welcome" docs to make the experience feel more human. …