Automatically test external links in docs on a regular basis

[webvictim]

As #11940 is removing external link checking on every PR due to false positives, we should come up with some other way to test external links in the docs on a semi-regular basis (I propose doing so daily).

Couple of options:

1. Some kind of cron job in Drone/GCB to run nightly and alert somewhere on failures
2. I think we already have an external service that monitors our website as a whole and reports back on SEO and other factors. Maybe this could also check links in the docs to make sure they resolve correctly?

[ptgott]

I'm going to work on nudging this toward a more specific plan in the next sprint. In the meantime, CCing @C-STYR and @jeffgaynor—it'd be great to get your thoughts on this. Thanks!

[ptgott]

I've talked to @C-STYR and @jeffgaynor on the Web team. We are currently using Semrush to manually check for 404ing external links on our production docs site on a weekly basis. The Web team will look for ways to check external links in a more automated fashion and for pre-production changes, but we're treating this as a low-priority issue since we have an existing solution.