How can Bevy's documentation be improved?

Newcomers are often a bit lost when trying to learn how to idiomatically work with assets and handles.

The most common stumbling block is "how do I change a sprite once loaded?".

We should add two sibling examples, one for sprites, and one for meshes:

• show how to query for sprite / mesh handles, and change what they point to
• show how to programatically change the properties of a sprite / mesh (e.g. color, reflectance, translucency or so on)
• show how to mutate the underlying asset to e.g. add more vertexes