ci: Add automated check for broken links

[thanethomson]

Follow-up to #25.

We need a check that runs regularly in CI (e.g. at least once daily) that crawls all deployed versions (even those not accessible from the nav) of the docs site and notifies us of any broken links.

Failure of the workflow is sufficient notification for now.

[andynog]

I think it's a good idea. Should the notification be an email sent to the team ?

Also, there might be false positives in the report since the tool we use to check the broken links cannot detect things like URL fragments properly.