[Doc] Improve diffusion generation parameter docs for online serving (#2051)

Signed-off-by: samithuang <285365963@qq.com>
Signed-off-by: Samit <285365963@qq.com>
Signed-off-by: gcanlin <canlinguosdu@gmail.com>
Co-authored-by: Zeyu Huang | 黃澤宇 <11222265+fhfuih@users.noreply.github.com>
Co-authored-by: gcanlin <canlinguosdu@gmail.com>

docs/.nav.yml

@@ -6,6 +6,7 @@ nav:
     - getting_started/installation/*
   - Serving:
     - OpenAI-Compatible API:
+      - Diffusion Chat API: serving/diffusion_chat_api.md
       - Image Generation: serving/image_generation_api.md
       - Image Edit: serving/image_edit_api.md
       - Text to Speech: serving/speech_api.md