Change linting workflow to only check modified files

I forget if this has come up before, so I'm creating an issue here for discussion.

Would it make sense to change the linting workflow to only run urlcheck on the files that were modified as part of a PR?

The motivation here is: we have a large and growing list of links to check in the content of our site. The urlcheck job already takes a long time to run. Checking every single link on the site for trivial changes seems... excessive. It also causes confusion for PR submitters when something they didn't touch fails the workflow, i.e., "hey, do I need to fix that expired job link to get my new event announcement merged???"

One upshot of the current method is that it provides a somewhat routine check for all the links and provides quick feedback to an attentive person. If we were to change the scope of the linting workflow, we should probably add a new cron-triggered workflow to urlcheck all the links on a periodic basis, maybe weekly or monthly, but then some group of people would need to be alert for failures.

Start by locating the existing linting workflow and its urlcheck job. Determine how modified PR files are selected and define a scheduled full-site check; done means PR checks avoid unrelated links while periodic checking still covers all links.