Add textlint to enforce writing guide

[jp-knj]

Description (required)

This PR adds and configures textlint to enforce the Astro Japanese writing guide for our documentation. It introduces a new prh.yml file with terminology rules (e.g., 標準表記の統一やコロン「：」の検出) and updates the .textlintrc.ja.json configuration to load those rules. contributors will receive automated feedback on terminology usage and formatting consistency in Japanese docs, reducing manual proofreading errors and ensuring a unified style.

📁 Directory Structure

.
├── .textlintrc.ja.json                ← (new) Textlint config for Japanese rules
├── prh.yml                            ← (new) proofreading dictionary (日本語用ルール)
├── package.json                       ← (modified) adds `lint:ja` script, textlint dependencies
└── src
    └── content
        └── docs
            └── ja
                └── …                  ← (existing) Japanese‐language MDX/MD files

Related issues & labels (optional)

• Closes: Add textlint CI to enforce Japanese translation style guide #11665
• Previous PR: Add textlint to enforce style guide #11691
• Suggested label: i18n

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