docs: cut over to Nextra and remove IslandJS

[codeacme17]

Parent

#73

What to build

Make Nextra the production documentation site after all content slices have landed. Readers should keep working URLs, language navigation, interactive examples, analytics, and static assets while the obsolete IslandJS and NextUI toolchain disappears from normal installs and builds.

Acceptance criteria

• A route inventory confirms that all English and Chinese public pages resolve or redirect intentionally.
• Production static build and hosting behavior are verified, including nested routes, assets, audio files, metadata, search, and the not-found page.
• Existing analytics behavior is retained or deliberately replaced and verified in the Nextra application.
• Default documentation commands use Nextra and no longer invoke IslandJS.
• IslandJS, NextUI, Framer Motion, generated legacy output, and documentation-only packages that are no longer imported are removed.
• The package, example, and Nextra documentation builds pass from a frozen install.

Blocked by

• docs: migrate bilingual guides and site navigation to Nextra #80 — docs: migrate bilingual guides and site navigation to Nextra.
• docs: migrate controller component documentation and examples #81 — docs: migrate controller component documentation and examples.
• docs: migrate visualization and container documentation #82 — docs: migrate visualization and container documentation.
• docs: migrate hook documentation and live audio examples #83 — docs: migrate hook documentation and live audio examples.