doc: hide Zhihu link from English docs

[qinxuye]

Summary

• hide the Zhihu blog card from the English docs index page
• keep the Zhihu card visible in zh-cn docs builds
• preserve the existing zh-cn navbar Zhihu icon behavior

Validation

• git diff --check
• python -m py_compile doc/source/conf.py
• python -m sphinx -b html -q doc/source /tmp/xinference-doc-en-zhihu-check2
• READTHEDOCS_LANGUAGE=zh-cn python -m sphinx -b html -q -D language=zh_CN doc/source /tmp/xinference-doc-zh-zhihu-check
• verified English index.html has no zhihu.com/org/xorbits or "Read our blogs"
• verified zh-cn index.html keeps zhihu.com/org/xorbits and "阅读知乎博客"

[gemini-code-assist]

Code Review

This pull request introduces a mechanism to conditionally filter out Chinese-specific documentation elements (marked with the class zh-cn-only) when building the documentation for non-Chinese languages. This is implemented via a custom Sphinx extension hook in doc/source/conf.py and applied to a Zhihu link in doc/source/index.rst. The review feedback suggests a safer way to remove nodes from the docutils tree to avoid potential ValueError exceptions if a node is already removed.

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[OliverBryant]

LGTM