[docs] fix Ascend NPU docs link

[Ruazzm]

What does this PR do?

Fixes #10527.

Updates the Chinese README's Ascend NPU documentation entry to the current Read the Docs URL. The previous ascend.github.io link now returns 404, while the new ascend.readthedocs.io link opens the LLaMA-Factory Ascend documentation.

Before submitting

• Did you read the contributor guideline?
• Did you write any new necessary tests? This is a README link-only change; no code tests are needed.

Validation

• git diff --check upstream/main..HEAD
• Verified https://ascend.readthedocs.io/zh-cn/latest/sources/llamafactory/ returns HTTP 200

[gemini-code-assist]

Code Review

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