Document the thumbnail safe area

There's no documentation of the safe area for event/scene thumbnails — what gets cropped on cards, heroes and lists. Creators find out by trial and error.

Document the safe zone with dimensions and an overlay/example image. Companion UX: the event submission uploader should show the same overlay (tracked in decentraland/sites).

Start by locating the event and scene thumbnail documentation in the docs repository and review how cards, heroes, and lists crop images. Document the safe-zone dimensions with an overlay or example image; the uploader overlay is tracked separately in decentraland/sites.

Written by the indexing model from the issue text.