Migration Guide 0.14 -> 0.15 missing stuff

Some missing migration hints I found so far:
* MapEntitiesResource is gone
* ImageScaleMode -> NodeImageMode/ SpriteImageMode (and now part of ImageNode/Sprite)

(I think I found more, but I only now thought about creating an issue, I will fill in the gaps)

Start with the Bevy 0.14→0.15 migration guide and review the listed changes: MapEntitiesResource, ImageScaleMode, NodeImageMode, SpriteImageMode, ImageNode, and Sprite. Confirm the migration details and add the missing hints, including any additional gaps found; done means the guide covers the relevant API changes.

Written by the indexing model from the issue text.