Add link previews

## What changed

- `public/index.html`: Open Graph and Twitter card tags, so links to the site show the title, a short description and the Pads screenshot.

## Notes

- First merge after the site joined the deploy pipeline, so this is the first deploy of mpckit.monoexp.com.

## Testing

- Served `public/` locally and checked the page still renders the same; the tags are in the head only.