Add dead link checker to CI

[holly-cummins]

We should not be making our users identify and report dead links for us (for example, most recently #1987 and #2093 and quarkusio/quarkus#42688 (comment)). Computers can do this job more effectively than people, and unlike people, they don't become disappointed in us when they find dead links.

We want to catch new dead links when they're introduced, and alert maintainers when existing links stop working. Getting that exact behaviour is non-trivial, but we can get close to it by failing the CI for problems with internal links, and raising a defect if external links stop working. It's possible that a PR would introduce a new external link that never worked at all, but it's more usual that links work when they're created, and then stop working.

The http://hub.quarkiverse.io and http://quarkus.io/extensions pages both use this model, so we should be able to copy their implementations.