Refactor: Move RAG components to dedicated top-level module (#3222)

* Move RAG components to top-level module

- Create src/crewai/rag directory structure
- Move embeddings configurator from utilities to rag module
- Update imports across codebase and documentation
- Remove deprecated embedding files

* Remove empty knowledge/embedder directory

src/crewai/rag/__init__.py

@@ -0,0 +1 @@
+"""RAG (Retrieval-Augmented Generation) infrastructure for CrewAI."""