docs: zh_CN: add iio index.rst translation

This patch translates Documentation/iio/index.rst into Chinese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Reviewed-by: Huacai Chen <chenhuacai@kernel.org>
Link: https://lore.kernel.org/r/20210121033302.558935-3-siyanteng@loongson.cn
Signed-off-by: Jonathan Corbet <corbet@lwn.net>