This adds a CI job for running the new PR link checker for documentation.

[terraform-website PR 1574](https://github.com/hashicorp/terraform-website/pull/1574)
added a new link checking CI job specifically for warning about broken links in
pull requests. This link checker is optimized for:

- Running (relatively) quickly.
- Only reporting on files that were changed in the current PR, to avoid spamming
  you with problems you had nothing to do with.
- Being transparent and simple to maintain. (Note that this is in conflict with
  minimizing false positives/negatives! We try to give very few of both, but
  completely eliminating them would result in an unaffordable maintenance
  burden. We expect that some PRs will be merged with this job red.)

The tool is somewhat specific to our Middleman site builder, and we expect it
will be replaced or obviated in the transition to the Next.js platform... but in
the meantime, it should help make documentation sligh...