Fix docs indexing with sitemap, canonical URLs, and root redirects

[tudddorrr]

Docs indexing fixes

• Added a sitemap at /sitemap.xml listing every docs page so search engines can discover them.
• Added canonical URLs to docs pages to prevent duplicate-content issues.
• Pointed robots.txt at the new sitemap.

Root and legacy redirects

• Redirect / to the default docs version (/1.x).
• Keep old /intro and /docs/* links working by redirecting them to the versioned docs.
• Removed the now-redundant intro redirect route.

Static generation and serving

• Include the sitemap in prerendered paths and skip the root redirect during the SPA fallback prerender.
• Serve assets with trailing slashes dropped and let the worker handle requests to /.