Demonstrate how to build a TextureAtlas from a collection of images

## How can Bevy's documentation be improved?

Sometimes users don't begin with a single sprite sheet: instead, they want to build one (or an underlying texture atlas) from a collection of images.

Figuring out how to do so is quite challenging. I seem to remember modifying the [texture_handles](https://docs.rs/bevy/0.9.1/bevy/sprite/struct.TextureAtlas.html) field directly, but this was a) hacky and b) very hard to discover.