Expanded examples in relation to texture atlases and sprite sheets to show how to use it in other systems

https://github.com/bevyengine/bevy/blob/latest/examples/2d/texture_atlas.rs
https://github.com/bevyengine/bevy/blob/latest/examples/2d/sprite_sheet.rs

I'm not sure exactly how to really use them within a project. In these examples the texture atlas is created and used within the same system, but I've been asking around for what the correct way to use the texture atlas in other systems is and I haven't had answers that have made it clear. If people know they aren't telling me, and it feels like it should be something so basic that it's in the examples, I just don't know the correct way to do it, it feels wrong the way I did it (my way no longer works, I must have saved it after making a breaking change and I don't know why I'm getting an error).

I would really like to see what the standard way to create a texture atlas and then use it elsewhere is.

Start with examples/2d/texture_atlas.rs and examples/2d/sprite_sheet.rs to understand how the atlas is currently created and used within one system. Expand the examples to demonstrate creating the atlas and using it from other systems, then run the examples to verify the documented workflow.

Written by the indexing model from the issue text.