docs: add Japanese (日本語) README

[RKasai127]

Description

Adds a Japanese translation of the project README as README.ja.md.

Spec Kit already has a Simplified Chinese translation (#3740) but no Japanese one, so this follows the same pattern.

New file

• README.ja.md — a full Japanese translation.
• README.md — added 日本語(japanese) to the language switcher under the badges.
• README.zh-CN.md — added 日本語(japanese) to its language switcher as well.

All code blocks, CLI commands, slash-command names, badges, image paths, and link URLs are unchanged — only prose and inline shell comments were translated.

Testing

This is a documentation-only change with no impact on CLI behavior or slash commands, so the checkboxes below don't apply.

• Tested locally with uv run specify --help — N/A (docs-only change)
• Ran existing tests with uv sync && uv run pytest — N/A (docs-only change)
• Tested with a sample project (if applicable) — N/A

Manual verification performed instead:

• Confirmed every Table of Contents anchor resolves to the correct (translated) heading.
• Confirmed section count, order, and links match the English README 1:1.

AI Disclosure

• I did use AI assistance (describe below)

I used Claude Code to produce an initial draft translation, then reviewed and corrected it myself as a native Japanese speaker — rewriting phrasing to read naturally rather than as a literal machine translation, and checking terminology and links against the English source section by section.

Note

If documentation/translation contributions like this aren't something the project wants to take on right now, please feel free to close this — no hard feelings, and thanks for all the work maintaining Spec Kit!