docs/zh_CN: add virt index translation

Add virt to .../zh_CN/index and translate it into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/b28d3a530fe0c3635e7be2462fd14a4ae18f0220.1628212777.git.siyanteng@loongson.cn


Signed-off-by: Jonathan Corbet <corbet@lwn.net>