docs/zh_CN: add riscv index.rst translation

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

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Link: https://lore.kernel.org/r/20210228191054.6048-5-siyanteng@loongson.cn


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