Document bevy_gizmos (#8186)

# 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.

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Co-authored-by: James Liu <[email protected]>

Author: tim-blackbird