完善文档

docs/rag/overview.md

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+# RAG
+
+## 架构
+
+```mermaid
+flowchart LR
+    Query --> Understanding
+    Understanding --> BM25
+    Understanding --> Vector
+    Understanding --> Graph
+    BM25 --> Fusion
+    Vector --> Fusion
+    Graph --> Fusion
+    Fusion --> Reranker
+    Reranker --> Compression
+    Compression --> LLM
+```
+
+## 检索
+
+采用 Hybrid Retrieval：
+
+- Keyword
+- BM25
+- Vector
+- Graph
+- Metadata Filter
+
+最终通过 Reranker 与 Context Compression 提高上下文质量。