remove and redirect files

Author: laujan

articles/ai-services/content-understanding/toc.yml

@@ -76,9 +76,6 @@ items:
     - name: 🆕 Prebuilt analyzers
       displayName: analyzer, templates, document, text, images, video, audio, multimodal, visual, structured, content, field, extraction
       href: concepts/prebuilt-analyzers.md
-    - name: Retrieval-augmented generation (RAG)
-      displayName: RAG, retrieval, augmented, generation, knowledge, base, search, index, vector
-      href: concepts/retrieval-augmented-generation.md
 - name: Tutorials
   items:
     - name: Create a custom analyzer