Add a toon shader

## What problem does this solve or what need does it fill?

Toon shading (also called cel shading) is a very common and popular effect for stylized games, discretizing colors to create an effect like that seen in Windwaker.


## What solution would you like?

We create and maintain a high quality example teaching users how to do this.

## What alternative(s) have you considered?

We could add a configurable built-in way to do this.

I think that an example is probably more valuable here, as the level of configurability needed is super high. If it turns out to be popular and feasible, we can later change the example into a feature.

No file, test, or entry point is named. Start by reviewing Bevy’s existing examples and shader documentation to determine where a maintained toon-shading example belongs; done means a high-quality example demonstrates the effect and its scope is documented.

Written by the indexing model from the issue text.