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

Switch the Elements docs site from hash-based to path-based SPA routing
to match the Core site (no-js.dev). Adds <base href="/">, sets
useHash: false, updates syncSidebar/syncPrevNext to use
location.pathname and popstate, and creates 404.html as a copy of
index.html for the GitHub Pages deep-link fallback.

Author: ErickXavier