docs: update Chinese NPU docs link

[he-yufeng]

Summary

• replace the stale Ascend NPU docs link in the Chinese README with the current ReadTheDocs NPU page
• keep the English README and the lower installation section unchanged

To verify

• Select-String -Path README_zh.md -Pattern 'llamafactory.readthedocs.io/zh-cn/latest/multibackend/npu/index.html'
• git diff --check

Closes #10527

[gemini-code-assist]

Code Review

This pull request updates the documentation link for the Ascend NPU framework in the Chinese README (README_zh.md) to point to the new location on Read the Docs. There are no review comments, and I have no additional feedback to provide.

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