docs/zh_CN: Add a entry in Chinese glossary

To avoid confusion with the term "entry," the glossary stipulates
that in contexts related to page tables, "page table entry" will be
translated as "页表项," while "entry" will be translated as "表项."

Signed-off-by: Pengyu Zhang <zpenya1314@gmail.com>
Link: https://lore.kernel.org/r/20241016141801.25487-5-zpenya1314@gmail.com
Reviewed-by: Yanteng Si <si.yanteng@linux.dev>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>