Add example for asset dependencies

## How can Bevy's examples be improved?

https://docs.rs/bevy/latest/bevy/asset/struct.LoadedAsset.html

For my project I have been relying on systems to do nested loading of assets, but asset dependencies simplify this greatly. However, Bevy currently has no examples regarding asset dependencies (which is probably why I never noticed the feature haha).

Bevy should add an example(s) of the asset dependency system.