Adds a helpful 404 page for the docs site with:
- Large 404 indicator
- Friendly message explaining the page wasn't found
- Quick navigation cards to get back on track:
  - Docs Home
  - Get Started guide
  - Learn section
  - Search

Fixes WEB-89

Vercel automatically serves 404.html from build output for unmatched routes

- Rename pages/404.mdx to pages/*.mdx so Vocs treats it as catch-all route
- Update build script to copy generated 404 to dist/404.html for Vercel

Vocs generates "/*" for *.mdx catch-all routes but checks for "*" in
routes.tsx, causing client-side navigation to show the default NotFound
component instead of the custom 404 page. This adds a Vite plugin to
patch the route matching at build time.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

- Rename *.mdx to 404.mdx so vocs builds it to dist/404/index.html
- Update build script to copy from dist/404/index.html
- Update vocs plugin to redirect unknown routes to /404 using Navigate

The catch-all *.mdx approach didn't work because vocs doesn't prerender
catch-all routes. This approach uses a regular /404 page that gets built
normally, then copied to 404.html for Vercel, with client-side routing
redirecting to /404.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

  Remove complex imports (icons, Card components) to test if they were
  causing silent prerender failures.

  Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>