docs: add Japanese README

[eltociear]

• I am sure that the code is update-to-date with the dev branch.

I created Japanese translated README.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (cdcec59) 76.23% compared to head (cf78c81) 22.72%.
Report is 6 commits behind head on dev.

Files	Patch %	Lines
...ets-ui/src/controllers/editor-bridge.controller.ts	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              dev     #857       +/-   ##
===========================================
- Coverage   76.23%   22.72%   -53.52%     
===========================================
  Files          85      718      +633     
  Lines        4297    41480    +37183     
  Branches      879     8586     +7707     
===========================================
+ Hits         3276     9427     +6151     
- Misses        904    27277    +26373     
- Partials      117     4776     +4659     

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[github-actions]

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[jikkai]

Thank you for the Japanese translation! We appreciate your help. Looking forward to working together in the future.

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日本語の翻訳にありがとうございます！お力添えに感謝しています。今後もよろしくお願いします。