docs(zh-CN): sync LlamaHub API example

[ShawnSiao]

Summary

Sync the Chinese LlamaHub example with the current LlamaIndex integration setup.

Change rationale

English source commit eede97f adds the Hugging Face embedding package, reads HF_TOKEN from the environment, selects the provider automatically, and prints the completion response.

Changes

• Updated units/zh-CN/unit2/llama-index/llama-hub.mdx.
• Added installation of llama-index-embeddings-huggingface.
• Replaced the hard-coded token placeholder with .env loading and HF_TOKEN lookup.
• Added provider="auto" and explicit response printing.
• Translated the two new reader-facing code comments.

Verification

• Ran git diff --check.
• Confirmed MDX code-fence and image counts are unchanged.
• Parsed the embedded Python example successfully with ast.parse.
• Confirmed no open overlapping PR for this page before submission.

Related to #152.