Link check internal links only

[lucyb]

We've had the new link checking process in place for a little while now and it appears to fail often. The link checker was recently configured to ignore all 50x errors and 429 errors (PR #1671). However, transient errors when checking the links still happen frequently. This causes false positive alerts and makes it more likely people will ignore them, and generally just creates more work for people.

Therefore, I propose that we only check the validity of links for the docs.opensafely.org domain itself. Potentially, we could expand this to other sites that we control (since we're able to control any errors we receive from our own sites).