docs: add Haystack RAG example (#615)

Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>

mkdocs.yml

@@ -79,6 +79,7 @@ nav:
     - Chunking:
       - "Hybrid chunking": examples/hybrid_chunking.ipynb
     - RAG / QA:
+      - "RAG with Haystack": examples/rag_haystack.ipynb
       - "RAG with LlamaIndex 🦙": examples/rag_llamaindex.ipynb
       - "RAG with LangChain 🦜🔗": examples/rag_langchain.ipynb
       - "Hybrid RAG with Qdrant": examples/hybrid_rag_qdrant.ipynb