📚 docs(readme): add Japanese translation of README

[yuppe44]

Summary

• Add README_ja-JP.md with full Japanese translation
  of the project README
• Add Japanese language link to the language switcher
  in README.md and README_zh-CN.md

Notes

• Structure and content are aligned with the existing
  Chinese translation (README_zh-CN.md)
• Documentation links point to the English docs
  (./docs/en/) as Japanese docs do not exist yet