🔧 Add LLM prompt file for Japanese, generated from the existing translations (#14545)

Co-authored-by: Maruo.S <raspi-maru2004@outlook.jp>

docs/ja/llm-prompt.md

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+### Target language
+
+Translate to Japanese (日本語).
+
+Language code: ja.
+
+### Grammar and tone
+
+1) Use polite, instructional Japanese (です/ます調).
+2) Keep the tone concise and technical (match existing Japanese FastAPI docs).
+
+### Headings
+
+1) Follow the existing Japanese style: short, descriptive headings (often noun phrases), e.g. 「チェック」.
+2) Do not add a trailing period at the end of headings.
+
+### Quotes
+
+1) Prefer Japanese corner brackets 「」 in normal prose when quoting a term.
+2) Do not change quotes inside inline code, code blocks, URLs, or file paths.
+
+### Ellipsis
+
+1) Keep ellipsis style consistent with existing Japanese docs (commonly `...`).
+2) Never change `...` in code, URLs, or CLI examples.
+
+### Preferred translations / glossary
+
+Use the following preferred translations when they apply in documentation prose:
+
+- request (HTTP): リクエスト
+- response (HTTP): レスポンス
+- path operation: パスオペレーション
+- path operation function: パスオペレーション関数
+
+### `///` admonitions
+
+1) Keep the admonition keyword in English (do not translate `note`, `tip`, etc.).
+2) If a title is present, prefer these canonical titles:
+
+- `/// note | 備考`
+- `/// note | 技術詳細`
+- `/// tip | 豆知識`
+- `/// warning | 注意`
+- `/// info | 情報`
+- `/// check | 確認`
+- `/// danger | 警告`