Docs: Usage of nested schema is not explained

[yn-coder]

Summary

In the documentation article about using Advanced Alchemy (AA) in the Litestar framework, the Author model is utilized, but the Book model remains unused. It is important to explain how to use a nested schema to handle this referenced data.

https://advanced-alchemy.litestar.dev/latest/usage/frameworks/litestar.html

https://github.com/litestar-org/advanced-alchemy/blob/main/docs/usage/frameworks/litestar.rst