Check links on build

[nate-double-u]

We should check that links in our site don't 404 when we do a PR.

If we agree, here is an option:

• It looks like Netlify has a plugin that could handle this: https://github.com/munter/netlify-plugin-checklinks#readme
  (it's listed on the plugins page https://app.netlify.com/teams/dex-8nqqgjy/plugins, but I don't want to try it w/o having a discussion first -- I'm not sure what licenses we want our tools to have, or indeed if this completely solves the issue of highlighting broken links)

I'm happy for other suggestions.

[nate-double-u]

/cc @justaugustus @sagikazarmark @celestehorgan @zacharysarah

[nate-double-u]

I've tested the plugin with a hugo-netlify-starter fork:
nate-double-u/testing-template#4

[nate-double-u]

All I needed to do was install the plugin, and add the plugin info to the netlify.toml file.

[nate-double-u]

I'm not super happy that when the link checker fails, all checks fail and that merging isn't blocked...

I'd rather the link check come up as a separate line item that has a detail link like the others so the committer doesn't have to open the Netlify Deploy logs to get the details. But that may be my own lack of knowledge of Netlify setup (is there a way to add a check to the list I'm missing?)

[sagikazarmark]

Not sure how the plugin works, but we could use a GitHub Action to check links. I'm pretty sure that either the plugin or a similar tool could validate links in the generated HTML.

[nate-double-u]

I think I'd rather use a GitHub action, the Netlify plugin seems pretty opaque. I'll do some research on the GitHub action and see if it's got a better developer experience.

[nabokihms]

Added a Makefile command to check links
#172

The tool itself is pretty popular, probably we can just run this check on CI periodically.