Japanese translation for 3.1/usingthymeleaf.md and thymeleaf31whatsnew.md

[MasatoshiTada]

We ( I and @bufferings ) finished translating 3.1/usingthymeleaf.md and thymeleaf31whatsnew.md into Japanese. We'd appreciate it if you publish these in thymeleaf.org.

Issue -> https://github.com/thymeleaf/thymeleaf-docs/issues/90

[ultraq]

Closing/opening to trigger the new GitHub Actions build checks

[ultraq]

Thank you for taking the time to translate these docs - I'll get these up on our website very soon 🙌

[ultraq]

Published and added links from the docs page at https://www.thymeleaf.org/documentation.html

• https://www.thymeleaf.org/doc/articles/thymeleaf31whatsnew_ja.html
• https://www.thymeleaf.org/doc/tutorials/3.1/usingthymeleaf_ja.html

[MasatoshiTada]

Thank you so much😆