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

* 🌐 Add Japanese translation for `docs/ja/docs/tutorial/dependencies/sub-dependencies.md`. PR [#1959](https://github.com/tiangolo/fastapi/pull/1959) by [@SwftAlpc](https://github.com/SwftAlpc).
* 🌐 Add Japanese translation for `docs/ja/docs/tutorial/background-tasks.md`. PR [#2668](https://github.com/tiangolo/fastapi/pull/2668) by [@tokusumi](https://github.com/tokusumi).

We welcome pull requests for your language.

### Translations

- [日本語 (Japanese)](docs/ja/README.md)
- [简体中文 (Simplified Chinese)](docs/zh-CN/README.md)
- [Español (Spanish)](docs/es/README.md)
- [Português (Brazilian Portuguese)](docs/pt-BR/README.md)
- [Français (French)](docs/fr/README.md)

* Make sure the new link is in the correct category (e.g. "Podcasts") and language (e.g. "Japanese").
* A new link should be at the top of its list.
* The link URL should work (it should not return a 404).
* The content of the link should be about FastAPI.
* The new addition should have these fields:

    public static final String ITEM_VALUE = "value";

    /** Item property key for name field. */
    public static final String ITEM_NAME = "name";

    /** Character encoding constant for Japanese MS932. */
    public static final String MS932 = "MS932";

    // ============================================================
    // Default Configuration Values