[Doc] refactor diffusion doc (#753)

Signed-off-by: zjy0516 <riverclouds.zhu@qq.com>

docs/.nav.yml

@@ -26,17 +26,16 @@ nav:
   - Configuration:
     - configuration/README.md
     - configuration/*
-  - Diffusion Acceleration:
-    - Overview: user_guide/diffusion_acceleration.md
-    - Acceleration Methods:
-      - TeaCache: user_guide/acceleration/teacache.md
-      - Cache-DiT: user_guide/acceleration/cache_dit_acceleration.md
-      - Parallelism Acceleration: user_guide/acceleration/parallelism_acceleration.md
   - Models:
     - models/supported_models.md
   - Features:
     - Sleep Mode: features/sleep_mode.md
-    - CPU Offloading for Diffusion Model: features/cpu_offload_diffusion.md
+    - Diffusion Features:
+      - Overview: user_guide/diffusion_acceleration.md
+      - TeaCache: user_guide/diffusion/teacache.md
+      - Cache-DiT: user_guide/diffusion/cache_dit_acceleration.md
+      - Parallelism Acceleration: user_guide/diffusion/parallelism_acceleration.md
+      - CPU Offloading: user_guide/diffusion/cpu_offload_diffusion.md
 - Developer Guide:
   - General:
     - contributing/README.md