Consolidate changelog files and improve translation workflow

[modernchina123]

Summary

This PR consolidates the changelog structure across all languages (English, Chinese, Korean) by merging 2025 and 2026 entries into a unified changelog.mdx file, updates navigation configuration, and improves the translation workflow to preserve front matter metadata.

Changes

📝 Changelog Consolidation

• Renamed files: 2025.mdx → changelog.mdx for all three languages (en, cn, ko)
• Merged content: Combined 2025 and 2026 changelog entries into a single file
• Updated filters: Added year information to month labels (e.g., "January 2026", "2025 年 12 月")
• Fixed formatting: Removed incorrect markdown code block markers that caused rendering issues

🗂️ Navigation Updates

• Updated docs.json to reflect the new unified changelog structure
• Changed Chinese tab name from "更新日志" to "变更日志" (Changelog)
• Unified group names across all languages

🔧 Translation Workflow Improvements

• Enhanced translate.js: Added explicit front matter preservation logic
• Added comments: Documented that YAML front matter should not be translated
• Updated system prompts: Added explicit instructions to LLM to preserve front matter
• Workflow documentation: Added comments in GitHub Actions workflow

🧹 Cleanup

• Removed temporary documentation files (PR_DESCRIPTION.md, WHAT_CHANGED.md)

Files Changed

Modified:
  .github/workflows/auto-translate.yml     (+2 lines)
  docs.json                                (14 changes)
  translate.js                             (+21 lines)

Renamed:
  cn/changelog/2025.mdx → cn/changelog/changelog.mdx
  en/changelog/2025.mdx → en/changelog/changelog.mdx
  ko/changelog/2025.mdx → ko/changelog/changelog.mdx

Deleted:
  PR_DESCRIPTION.md
  WHAT_CHANGED.md

Impact

• ✅ All changelog entries are now in a single, unified file per language
• ✅ Improved maintainability with consistent file naming
• ✅ Better translation workflow that preserves metadata
• ✅ Fixed rendering issues in Chinese changelog
• ✅ Cleaner repository without temporary files

Testing

• Local preview verified (mint dev)
• All three language versions render correctly
• Filters and month labels display properly with year information
• Translation workflow tested via GitHub Actions