Refine Japanese docs links and styling - #46

Summary

• fix Japanese docs links so internal navigation consistently stays in docs/ja
• remove unnecessary fixed version mentions from Japanese docs while keeping release references accurate
• adjust the docs language toggle styling for better layout alignment

Details

• update internal links in Japanese docs pages to explicitly target ja/...
• replace outdated v0.9.9 / v0.9.8 mentions in general documentation text where the version should be omitted
• keep historical version references in changelog-style content intact
• refine the .lang-toggle layout in docs/index.html

Testing

• reviewed docs/ja pages for internal link targets
• confirmed current release download references point to 0.9.9.2
• verified the working tree is clean after commit