PR Doc #5443 broke the link checker because it includes a link to this
private repo:

https://github.com/paritytech/devops/issues/3502

This PR adds the `prdoc` folder to the excluded paths of the link
checker to ensure that historical PR Docs do not break the pipeline.

I think this makes sense over whitelisting the github link because we
probably expect that in a long enough time period, links from old PR
docs will start to break, but I don't think we intend to update those
old PR docs, or start whitelisting lots of urls.