docs/zh_CN: Add translation zh_CN/maintainer/rebasing-and-merging.rst

Add a new translation
  Documentation/translations/zh_CN/maintainer/rebasing-and-merging.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/040f93d0f773a0c9c8d6637701ba269d816a6385.1621243426.git.bobwxc@email.cn
Signed-off-by: Jonathan Corbet <corbet@lwn.net>