[feature] Broken link checker

It would be great if we had a plugin which automatically failed the build if we've introduced a broken link.

As originally submitted in https://github.com/apollographql/engine-docs/pull/138 by @evans, but with the intention of having it affect all of our docs repositories, not just `apollographql/engine-docs`.

Start by reviewing the original Apollo PR 138 and the documentation repositories it was intended to affect. Identify the plugin's integration point and define how a broken link should make a build fail; the work is done when this behavior applies consistently across the relevant docs repositories.

Written by the indexing model from the issue text.