♿️ fix: improve accessibility and i18n

[welpo]

Summary

This PR addresses #182 by enhancing the accessibility, internationalisation, and overall usability of the theme. Improvements have been made in several areas including the language switcher dropdown, theme switcher, and the copy code button.

Enhancements for Internationalisation

• Theme Switcher: Added ARIA labels and title elements to improve internationalisation support. The strings are saved in hidden HTML tags, as Zola cannot create JS files with Tera templates.
• Language Switcher: Included ARIA labels, title attributes, and set the "lang" attribute for appropriate voice/accent in screen readers.
• Copy Code Buttons: Incorporated ARIA labels and title tags, making them localizable.

Improvements in Accessibility

• Theme Switcher: Converted to a toggle.
• Language Switcher: Switched to summary/details HTML elements for better accessibility. The menu now opens on click, not hover.
• Copy Code Buttons: Made the buttons selectable by screen readers.

UI/UX Tweaks

• Theme Switcher and Language Switcher icons now change colour on hover to indicate interactivity.