Fix docs language switcher and Japanese README - #45

Summary

• fix Docsify sidebar alias precedence so Japanese pages load the Japanese sidebar
• improve the docs styling in docs/index.html to restore the intended appearance
• update the Japanese docs README to 0.9.9.2

Details

• ensure /ja/ routes resolve /ja/_sidebar.md before the default sidebar alias
• add lightweight custom docs styles for banners, blockquotes, code blocks, tables, and release download links
• align the Japanese docs landing page version badge and ZIP references with the 0.9.9.2 release

Testing

• verified the docs config changes are limited to docs/index.html
• verified the Japanese docs README now points to swift-csv-0.9.9.2.zip
• confirmed the branch is clean after commit