docs: Add Japanese README (awslabs#106)

* Add Japanese README (README.jp.md)

* Add cross-language links between README.md and README.jp.md

Add language selector line to the main README linking to the Japanese
translation, and a reciprocal link in README.jp.md back to English.
Also apply dprint formatting to README.jp.md.

---------

Co-authored-by: bonk-ai[bot] <bonk-ai[bot]@users.noreply.github.com>

Author: minorun365