docs/zh_CN: add riscv to zh_CN index

All the files in the riscv directory have been translated into
Chinese and it is time to add them to the zh_CN index.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/20210406070239.19910-9-siyanteng@loongson.cn


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