docs/zh_CN: rename email-clients.txt as email-clients.rst

Make it available for html etc docs.
Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Harry Wei <harryxiyou@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
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>