docs: add Japanese README

[eltociear]

I created Japanese translated README.

[gemini-code-assist]

Summary of Changes

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This pull request adds a Japanese translation of the README file, making the project more accessible to Japanese-speaking users. It also updates the English and Chinese README files to include a link to the new translation.

Highlights

• New Translation: This PR introduces a Japanese translation of the README, enhancing accessibility for Japanese-speaking users.
• README Updates: The English and Chinese README files were updated to include a link to the new Japanese README.

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[gemini-code-assist]

Code Review

This pull request introduces a Japanese translation of the README file. This is a great addition for making the project more accessible. My review identifies a few opportunities to improve the user experience for Japanese readers by updating external links to point to their corresponding Japanese versions. Please see the detailed comments.

[luispater]

Summary

This PR adds a new Japanese README (README_JA.md) and wires language navigation links from README.md and README_CN.md to the new document.

Key findings

No blocking or non-blocking issues found in this review.

Validation

• Confirmed scope is docs-only (README.md, README_CN.md, README_JA.md).
• Confirmed language switch links were updated to include Japanese.
• Confirmed Japanese guide/management/Amp links point to /ja/... paths.
• CI status is green for relevant checks; translator-path-guard was intentionally ignored for recommendation as requested.

Recommendation

Leaving a comment instead of approval because the automated reviewer confidence was not high enough to approve.

This is an automated Codex review result and still requires manual verification by a human reviewer.