Add textlint to enforce style guide

[jp-knj]

✨ What does this PR do?

This PR introduces textlint to enforce consistent writing style and terminology for localized documentation in:

• 🇯🇵 Japanese (ja)
• 🇨🇳 Simplified Chinese (zh-cn)

It uses per-language config files, not root-level .textlintrc.*.json, to keep the setup clean and scalable.

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📁 Directory Structure

astro/docs/
├── src/
│   └── content/
│       └── docs/
│           ├── ja/              ← Japanese translations
│           └── zh-cn/           ← Simplified Chinese translations
├── textlint/
│   ├── ja/
│   │   ├── config.json          ← textlint config for Japanese
│   │   └── prh.yml              ← Japanese terminology dictionary
│   └── zh-cn/
│       ├── config.json          ← textlint config for Simplified Chinese
│       └── prh.yml              ← Chinese terminology dictionary
├── scripts/
│   └── lint-text.mjs           ← Script to run textlint per language

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