Remove docs site contributing section

[version-1]

Remove the docs-site Contributing section

Summary

Temporarily remove the Contributing section from the published Docusaurus site while preserving the repository-level development documentation.

主な変更点:

• Remove the Contributing category from the docs sidebar.
• Delete the three English and three Japanese Contributing pages.
• Remove the unused Japanese sidebar translation label.
• Synchronize the English and Japanese docs-site README structure.

チケットへのリンク

ここはまだない

やったこと

• Removed Development Setup, Running Locally, and Testing from the published site.
• Kept general contributor wording and repository development documents unchanged.
• Corrected the docs-site README page inventory to describe the remaining 17 pages.

動作確認

• npm --prefix docs/site run build
• git diff --check
• jq empty docs/site/i18n/ja/docusaurus-plugin-content-docs/current.json
• Confirmed 17 Markdown pages remain in each locale.
• Confirmed no Contributing section references remain in docs-site sources.

レビュー & 動作確認 チェックリスト

• Sidebar - Confirm the Contributing category no longer appears.
• Localization - Confirm English and Japanese builds expose the same page set.
• Scope - Confirm repository-level development documentation remains available.

推奨テスト計画:

1. Open the English and Japanese docs-site navigation.
2. Confirm the Contributing category is absent.
3. Navigate through the remaining categories and confirm there are no broken links.

その他気になることや相談ごと

The removed pages remain recoverable from Git history if the section is restored later.

Requested by: @version-1