Update documentation to reflect changes in WebSocket API message formatting and knowledge base

- Updated the WebSocket API reference to improve clarity by removing unnecessary headings and emphasizing message types.
- Revised the index.md to specify 'chroma' as the knowledge base, enhancing the overview of the platform's architecture.

docs/content/index.md

@@ -120,7 +120,7 @@ flowchart LR
 | **后端** | FastAPI (Python 3.10+) |
 | **引擎** | Python, WebSocket, asyncio |
 | **数据库** | SQLite / PostgreSQL |
-| **知识库** |  |
+| **知识库** | chroma |
 | **部署** | Docker, Nginx |
 
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