Fix pagination links broken on gistpreview.github.io (#32)

The JavaScript that rewrites relative URLs for gistpreview.github.io
now handles dynamic content and SPA-style navigation:

- Skip already-rewritten links (starting with '?') to prevent
  double-rewriting when JS runs multiple times
- Use MutationObserver to catch dynamically added links
- Run on DOMContentLoaded as fallback for DOM timing issues
- Add longer delay retry (2s) for slow-loading content

Fixes #26

https://gisthost.github.io/?3581c145202388b43862c93176e9f8ad