Don't test github links

Unfortunately, we are being rate limited by GitHub, it seems, so our external link checker is getting nothing by Error 429's for now. Let's skip those links for now so we can get some useful link checking done and fix everything else, then when it's green we can try to fix GitHub links (and gitlab.com links with similar issues due to cloudflare).

For example: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/624625940