Add linkchecker job to check for links between our own domains

[nottrobin]

Links between our domains should work; but even then we also remove/rename/move things too =/ Sounds hard.

This is an interesting idea - worth checking if we can set up linkchecker to only check for links to domains we know we own. That should be a smaller set that we might be able to stay on top of.

From https://github.com/canonical-web-and-design/ubuntu.com/issues/11198#issuecomment-1102327224