Standardize README language filenames and links

[GaspardKirira]

Description

The Japanese README was added in PR #35, and a follow-up fix in PR #132 standardized:

• Filename: README-JP.md → README.ja.md
• Link in README.md pointing to the correct file

Acceptance criteria

• README.ja.md exists at repository root
• README-JP.md does not exist anymore
• README.md links to README.ja.md
• README.ja.md links back to README.md

Related PRs: #35, #132