Move link validator to nightly/weekly job

The link validator is currently run on PR validation, however it is far too susceptible for intermittent failures not caused by the PR in question.

We should probably move it to a nightly/weekly job. I guess the main question is how do we make sure we actually notice it when this job starts failing? Perhaps it should email the dev list?

Start by locating the PR validation workflow that currently runs the link validator and trace how its failures are reported. Move that check into a nightly or weekly job, then verify that failures reliably notify the development list or another defined channel.

Written by the indexing model from the issue text.