docs/zh_CN: add damon index tronslation

1) Translate .../vm/damon/index.rst into Chinese.
2) add damon into .../zh_CN/vm/index.rst

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/8e8c7651785f1ce20766bc1b3a4fc44faedb84bb.1642230669.git.siyanteng@loongson.cn


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