docs/zh_CN: rename stable_kernel_rules doc

docs/zh_CN: rename stable_kernel_rules doc

This patch renamed stable_kernel_rules.txt to stable-kernel-rules.rst
Make it available in htmldocs making.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Harry Wei <harryxiyou@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: TripleX Chung <xxx.phy@gmail.com>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Cc: Coly Li <colyli@suse.de>
Signed-off-by: Weiwei Jia <harryxiyou@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>