Bevy::Reflect Custom Attributes

## How can Bevy's documentation be improved?

Bevy Reflect's top level docs page and side bar make no mention or link to the Custom Attributes feature.
A mention or link to custom attributes should be added.
https://docs.rs/bevy/latest/bevy/reflect/index.html#

https://docs.rs/bevy/latest/bevy/reflect/attributes/struct.CustomAttributes.html

Start with Bevy Reflect's top-level documentation page and the CustomAttributes documentation at the linked docs.rs URLs. Locate the source for the Reflect docs page and sidebar, then add a mention or link to Custom Attributes; the work is done when the feature is discoverable from the top-level page and sidebar.

Written by the indexing model from the issue text.