Include shared docs in both trait and derive documentation

[alice-i-cecile]

I think it would make sense to split out the derive syntax section of the Component and Event docs into a separate markdown file, then #[doc = include_str!("…")] it in both the trait doc and the derive macro doc.

Originally posted by @SpecificProtagonist in #19523 (comment)

[theotherphil]

The same trick would make sense for (at least!) bevy_asset_macros.