revisit"linkcheck" test

The linkcheck test tests whether all the links in the documentation work, however for as long as I can remember this fails in ~100% of cases.

Here is an example of such a failure: https://gitlab.kwant-project.org/kwant/kwant/-/jobs/85564

The key lines are these:

(line  130) broken    http://glaros.dtc.umn.edu/gkhome/metis/metis/overview - HTTPConnectionPool(host='glaros.dtc.umn.edu', port=80): Max retries exceeded with url: /gkhome/metis/metis/overview (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f0248d45f28>: Failed to establish a new connection: [Errno 110] Connection timed out',))

If I browse to http://glaros.dtc.umn.edu/gkhome/metis/metis/overview I get the website and everything seems fine.

The linkcheck test has a very low specificity for detecting dead links. This is bad because in the UI we no longer see "all green", but because we know that this test often fails for "no reason" then we ignore the times when it fails, even if these indicate a true error.

I proposed to tweak the linkcheck parameters to make the test more specific.

If this does not work I propose to simply remove the check