docs(ja): add Japanese translations

[Kenzo-Wada]

Description

added japanese translation

Linked Issues

closes: #4514

Additional Context

[Copilot]

Pull Request Overview

This PR adds comprehensive Japanese translations for VitePress, enabling Japanese-speaking users to access the documentation in their native language.

• Adds Japanese translations for the complete documentation including guide and reference sections
• Updates configuration files to support Japanese locale
• Includes localized navigation, search, and UI elements

Reviewed Changes

Copilot reviewed 39 out of 39 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
docs/lunaria.config.json	Adds Japanese locale configuration for translation management
docs/.vitepress/config.ts	Adds Japanese locale to the main config and copy code button translation
docs/ja/config.ts	Creates Japanese-specific configuration with translated navigation and search options
docs/ja/index.md	Translates the homepage with Japanese hero content and features
docs/ja/guide/*.md	Translates all guide pages including getting started, routing, markdown extensions, etc.
docs/ja/reference/*.md	Translates all reference documentation including site config, runtime API, and default theme docs

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[brc-dd]

お疲れ様です！

Can you add the anchors to heading? They are currently needed for language switching to reliably work. For example:

# アセットの取り扱い

to

# アセットの取り扱い {#asset-handling}

You can check other translations for reference.

[Kenzo-Wada]

@brc-dd thx for the reviews!

i've added anchors at 24e1ed7 and f9d0dff, ready to re-reviewed!