docs: update and refine Japanese translations and navigation

[era760]

Description

This PR improves the Japanese translation files and establishes a clear language-switching navigation between the English and Japanese documentation.

Key Changes:

• Added Language Navigation: Added Other Languages: English | 日本語 quick-links at the top of both the English and Japanese versions of README and CONTRIBUTING files for seamless switching.
• Refined Japanese Translation:
  • Completely rewrote README_jp.md and CONTRIBUTING_jp.md to be more natural, concise, and professional (removed redundant notes, fixed typos, and completed unfinished sentences).
  • Updated outdated setup details (e.g., Tauri environment prerequisites).

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Motivation

The existing Japanese documentation was a bit outdated and contained some redundant or broken phrasing. By refining the translations and adding cross-linking between the English and Japanese files, we can provide a much better onboarding experience for Japanese-speaking contributors.

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Additional Context

I ensured that all local file links (using _jp.md rather than _ja.md to match the repository's historical file naming) are fully working and correct:

• README.md <--> README_jp.md
• CONTRIBUTING.md <--> CONTRIBUTING_jp.md

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