Add github action to find HTTP resources returning a 404 error code

[dstadulis]

This github action https://github.com/marketplace/actions/404-links should automatically test HTTP resources to ensure they don't return a 404 error code.

Pull Request Checklist

• [x] The documents updated are not in the docs/ directory. These files are synced from upstream repositories (lnd, lit, loop, pool and faraday), and should be updated in their parent repo.

[guggero]

I don't think we want merge commits in our repos. Can you rebase please?

[dstadulis]

Don't know what's happened with all these new commits -- going to close