Document the recipe for adding crate-relative asset source

## How can Bevy's documentation be improved?

Assets V2 added the ability to add multiple asset sources. One of the most useful use cases for this is for library crates that have their own asset collections. For example, a library of gizmos might have an assets folder for custom shaders; a library of mouse cursors might have assets for cursor images. Libraries like these would typically export a `Plugin` which registers their own assets; the root folder of these is a path which is relative to the crate root.

I suggest that the documentation for asset sources contain a standardized recipe for doing this: showing the best way to locate the asset folder relative to the crate and using that to register an asset source whose name is the crate name.

Either that, or an example.