reafator pipeline stage/step pipeline (#1368)

Signed-off-by: asukaqaq-s <1311722138@qq.com>

docs/.nav.yml

@@ -60,6 +60,7 @@ nav:
         - FP8: user_guide/diffusion/quantization/fp8.md
         - Int8: user_guide/diffusion/quantization/int8.md
         - GGUF: user_guide/diffusion/quantization/gguf.md
+      - Step Execution: user_guide/diffusion/step_execution.md
       - Parallelism Acceleration: user_guide/diffusion/parallelism_acceleration.md
       - CPU Offloading: user_guide/diffusion/cpu_offload_diffusion.md
       - LoRA: user_guide/diffusion/lora.md
@@ -91,6 +92,7 @@ nav:
       - design/feature/cache_dit.md
       - design/feature/teacache.md
       - design/feature/async_chunk_design.md
+      - design/feature/diffusion_step_execution.md
     - Module Design:
       - design/module/ar_module.md
       - design/module/dit_module.md