docs(zh): add missing ja/ and ko/ sections to docs directory tree in README_CN.md

[JiayuuWang]

Summary

The docs/ directory tree in README_CN.md was missing the ja/ (Japanese) and ko/ (Korean) sections. These sections exist in the actual repository under docs/ja/ and docs/ko/, and are correctly shown in the English README.md and partially in README_KR.md.

This fix adds the missing sections to make the Chinese README's directory listing accurate and complete, matching the actual repository structure.

Before: Only en/ and zh/ sections shown
After: All four sections (en/, ja/, ko/, zh/) shown, matching the real docs/ layout

Changes

• README_CN.md: Added ja/ and ko/ sections to the docs/ directory tree (lines 35–46)