feat: Add Japanese translation of README.md

[google-labs-jules]

This change introduces a Japanese translation of the repository's main README.md file. The new file, README.ja.md, provides a localized version of the project's description, installation instructions, and API usage examples. A link to the original English README has been included for easy navigation between the two versions.

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PR created automatically by Jules for task 6531671530896499945 started by @sashimikun

[google-labs-jules]

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