docs/zh_CN: add translations in zh_CN/dev-tools/gcov

Add new zh translations
* zh_CN/dev-tools/gcov.rst
* zh_CN/dev-tools/index.rst
and link them to zh_CN/index.rst

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Reviewed-by: Wu XiangCheng <bobwxc@email.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Reviewed-by: Fangrui Song <maskray@google.com>
Signed-off-by: Wu XiangCheng <bobwxc@email.cn>
Link: https://lore.kernel.org/r/20210415054304.GA26689@bobwxc.top
Signed-off-by: Jonathan Corbet <corbet@lwn.net>