`Image` docs are fairly impenetrable

## How can Bevy's documentation be improved?

The [`Image`](https://docs.rs/bevy/latest/bevy/prelude/struct.Image.html) type is very low-level, and not approachable for beginners.

For example:

1. What does the data field store, in what order, and how is it related to the `TextureFormat`?
2. What's the difference between `Size` and `TextureDImension`?
3. What does the `texture_descriptor` do and how does it differ from the `texture_view_desciptor`.
4. What does an `ImageSampler` do?
5. What does the "raw binary data" in `Image::new` refer to?