One of my pet peeves about technical documentation is dead links. There is nothing worse than having the answer to your question be a link that leads to the dreaded 404. Unfortunately, this is quite a common situation. Without some kind of automated link checking, eventually, any documentation that links to other resources will include stale links.
Fortunately checking for dead links does not have to be difficult. Iridium is a free and open source end to end testing tool, and when combined with a CI server like Travis CI, Iridium can be turned into a comprehensive automated link checking solution.