Consider adding hyperlink dead-link checking to example site

[mhansen]

Hi there, I just integrated https://github.com/untitaker/hyperlink, an intra-site link checker test that can run on CI, into Graphviz's Docsy site.

The checker is great, it immediately found 1000 dead links, it's very fast, and it doesn't use the network, so it's not flaky: see my testimonial: https://github.com/untitaker/hyperlink#testimonials.

I think perhaps this is worth baking into the Docsy platform by default -- perhaps starting by building the github action into the CI for docsy-example? Everyone struggles with dead links inside their websites, this is somewhere Docsy could add value with sensible defaults.

It was pretty straightforward to integrate on GitLab: https://gitlab.com/graphviz/graphviz.gitlab.io/-/blob/5dcfa637b7df17e3a1b821f3d7e9de8f5f82544b/.gitlab-ci.yml#L27 and it should be even easier on GitHub as there's a GitHub Action.