Document bevy_gizmos

[tim-blackbird]

Objective

Fix #8179

Solution

• Added #![warn(missing_docs)] and document all public items. All methods on Gizmos have doc examples.
• Expanded the docs on the module/crate. Some unfortunate duplication there :/
• Moved the methods from GizmoBuffer to be directly on Gizmos and made GizmoBuffer private. This means the methods on Gizmos will show up on its doc page.

[alice-i-cecile]

Once there's module docs this LGTM. Solid work so far, and I like the gratuitous use of doc examples.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[james7132]

Docs look good to me. The removal of GizmosBuffer as a pub struct is a breaking change though, as is the change from a alias to a actual struct.

[tim-blackbird]

Thanks for the review!
There are no breaking changes here since this API was added after 0.10 was released.

[james7132]

There are no breaking changes here since this API was added after 0.10 was released.

Ah yeah, you're right. Thanks for noting that.