docs(ko): move table of contents to top and add missing ja/ docs section in README_KR.md

[JiayuuWang]

Summary

Two structural issues fixed in README_KR.md:

1. Table of Contents displaced to middle of file: The ## 목차 (Table of Contents) section was located near the bottom of the file (around line 488), far from where readers expect to find it. This PR moves it to the top of the document, immediately after the language bar separator — consistent with the English, Chinese, and Japanese READMEs.

2. Missing ja/ section in docs directory tree: The docs/ directory tree listing was missing the ja/ (Japanese) language section. The actual repository contains docs/ja/ with 5 files, and all other READMEs list all four language directories. This PR adds the missing ja/ section to match the real repository structure.

Changes

• README_KR.md: Moved ## 목차 section from line ~488 to line 11 (after language bar)
• README_KR.md: Added ja/ section to docs/ directory tree (between en/ and ko/)