docs/zh_CN: Add translation zh_CN/maintainer/pull-requests.rst

Add a new translation
  Documentation/translations/zh_CN/maintainer/pull-requests.rst
and link it to zh_CN/maintainer/index.rst

Signed-off-by: Wu XiangCheng <bobwxc@email.cn>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/4774d1b7976678ce97c4356bd71509df0cec1ffc.1621243426.git.bobwxc@email.cn


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