Create GitHub Actions script to check for broken links

[sshine]

Currently several links in README.md are broken:

• https://tracks.exercism.io/haskell/master/unimplemented (master has become main)
• https://github.com/exercism/docs/blob/main/you-can-help/implement-an-exercise-from-specification.md (404)
• ...

It would be neat if we were notified if a link is broken for an extended period of time.