Add priority list to external link check task

As discussed in this thread: #577 (comment 921013527)

The Check for broken external links job produces a lot of output, and it's unclear how TWs should tackle it. Rather than skipping the check altogether, this MR proposes how to prioritize the link fixes.

It also reorganises some of the content in that section, and adds guidelines for what to do when you can't find a suitable replacement for a link that no longer exists.