docs/zh_CN: Add userspace-api/ebpf Chinese translation

Translate the following documents into Chinese:

- userspace-api/ebpf/index.rst
- userspace-api/ebpf/syscall.rst

Signed-off-by: Rui Li <me@lirui.org>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/cdd100d92752f76827fa7abfcd9903b857d71044.1666185911.git.me@lirui.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>