Improve documentation for `DeserializeWithRegistry` and `SerializeWithRegistry`

## How can Bevy's documentation be improved?

I think it's not very clear how to use these traits from their description. I only figured it out because of the unit tests inside `bevy_reflect`:

https://github.com/bevyengine/bevy/blob/4bbd37d4b052f4ed1db84a74a1e7bc20462a6943/crates/bevy_reflect/src/serde/mod.rs#L251-L333

For my game, I implemented deserialization using short names and allowing a string as shorthand for default deserialization:
https://github.com/simgine/simgine/blob/master/core/src/asset_manifest/reflected_component.rs

Scroll to the unit test to see how it works. Feel free to use it as an example in the Bevy docs 🙂

Start with the unit tests in crates/bevy_reflect/src/serde/mod.rs, especially the section covering DeserializeWithRegistry and SerializeWithRegistry. Compare those examples with the traits' current descriptions and review the linked reflected_component.rs example for short-name and string shorthand usage. Done means the documentation explains how to use both traits with representative examples.

Written by the indexing model from the issue text.