[docs] docs: fix diffusion docs structure and align ZH/EN toctree

[Zachary-wW]

Summary

• Remove redundant wan_tutorial/index.rst intermediate layer ("Diffusion Training Tutorial") from EN docs — toctree now lists sub-pages directly under "Diffusion Training" caption, consistent with other sections (LLM/VLM/VLA)
• Add missing ZH wan_tutorial/wan_packing.md (translated from EN) and add it to ZH toctree
• Align ZH LLM/VLM features structure with EN: replace flat feature entries in index.rst with 高级特性 <features_index> sub-pages (llm_tutorial/features_index.md, vlm_tutorial/features_index.md), matching the EN "Advanced Features" pattern