Fix external link checking: Put failing links to the ignore list

Currently, the nightly pipeline fails due to certain external dependencies that do not properly configure SSL certificates. Therefore, I extended the ignore list when checking external links.

@jandt-desy Can we go this way?

.gitlab-ci.yml

@@ -160,7 +160,7 @@ test:htmlproofer:external_links:
   script:
     - "bundle exec htmlproofer \
         --internal-domains localhost:4000 \
-        --url-ignore 'https://www.openstoragenetwork.org' \
+        --url-ignore '/www.openstoragenetwork.org/,/www.hzdr.de/,/indico4.twgrid.org/,/aai.helmholtz.de/,/cvs.data.kit.edu/' \
         --assume-extension public/"
   rules:
     - if: '$CI_PIPELINE_SOURCE == "schedule"'