RAG section improvements

Hi,

I've started reading the book and I have a few suggestions about the RAG section.

It is currently focused on the traditional semantic RAG with embedding. However in my experience teaching RAG, it's better to start with the broader definition (RAG = search engine + gen AI), on the example of a basic keyword search, and then introduce the common implementations (semantic RAG with embeddings, agentic RAG).

Would you accept a pull request with improvement proposals for this section?