Do not exclude domains from dead link checks

[martukas]

Documentation dead link checks should not exclude any domains.

How do you know it has to be done
Some of the links in our documentation may not lead to valid sites or pages. And we would not know this unless we checked them all manually. This is the one sub-task in #1313 that could not be satisfied. There are a dozen domains that are excluded from the documentation link checks. This is because those sites appear to block crawlers.

Starting points
Not sure of the mechanism or criteria for this blocking, but it maybe that it can detect the HTTP requests coming from CircleCI or Github servers. If we set up our own Jenkins CI on our own server (#1309 ), we may not be beholden to the same restrictions.