Rebase website from techdoc template

[MelbourneDeveloper]

TLDR;

Website now uses eleventy-plugin-techdoc as npm dependency. Fixed canonical URLs to dartnode.org.

What Does This Do?

• Rebases website to use eleventy-plugin-techdoc npm package instead of inline config
• Fixes all canonical/meta URLs from dartnode.dev to dartnode.org
• Adds npm run update:theme script for easy theme updates
• Removes redundant files now handled by plugin (feed.njk, sitemap.njk, robots.txt, llms.txt)

Brief Details?

• eleventy.config.js simplified - delegates to techdoc plugin
• site.json url changed to https://dartnode.org
• index.njk JSON-LD URLs updated to dartnode.org
• package.json adds update:theme script and eleventy-plugin-techdoc dependency

How Do The Tests Prove The Change Works?

• cd website && npm run build succeeds
• Built site verified against live dartnode.org - structure matches
• grep dartnode.org website/_site/index.html confirms correct domain (26 occurrences)
• Only remaining dartnode.dev is pub.dev publisher URL (correct)