feat(docs): enable path-based SPA routing via 404.html

[ErickXavier]

Summary

• Switches the Elements docs site from hash-based to path-based SPA routing, matching the Core site (no-js.dev)
• Adds <base href="/"> for correct relative URL resolution on deep paths
• Sets useHash: false in the NoJS router config
• Updates syncSidebar() and syncPrevNext() JS helpers to use location.pathname and popstate instead of location.hash and hashchange
• Updates prev/next navigation links from href="#/path" to href="/path"
• Creates docs/404.html as a byte-for-byte copy of docs/index.html for GitHub Pages deep-link fallback

Test plan

• Verify the docs site loads at the root URL (/)
• Navigate to a deep route (e.g., /accordion) and verify the page renders
• Refresh on a deep route and verify the 404.html fallback serves the SPA correctly
• Verify sidebar visibility toggles correctly (hidden on home, visible on component pages)
• Verify prev/next navigation links work and point to path-based URLs
• Verify mobile navigation links work