Document `bevy::gizmos`

[jakobhellermann]

How can Bevy's documentation be improved?

The gizmo module was added in #6529 but it is not documented right now: https://dev-docs.bevyengine.org/bevy/gizmos/gizmos/index.html

We should document

1. which type is the system param you want to use?
2. have a module-level example for usage
3. have per-method usage examples that are easy to copy paste. Especially for the methods returning a SphereBuilder or CircleBuilder it's not clear to be how they are intended to be used, just from the docs.