Warn about 404 links

[Volker-Weissmann]

If you do[desc](path/to/nonexisting.html), you obviously get a 404 link. I think running mdbook should either error out or at least print a warning so that it's caught in testing/CI/CD.

For example, I just found some 404 links in the lalrpop book.

[ehuss]

mdbook-linkcheck is the common solution for adding link checking. Otherwise, I think this is mostly a duplicate of #431, so closing in favor of that.