Improve link checking

• Fix several bad internal links
• Update html-proofer-plugin which is used for checking links to latest version 1.2.1
• Link checker settings:
  • Fix comments/documentation
  • Add urls to ignore
  • Throw error after checking all links (not at first bad link). This will hopefully reduce number of pipeline runs overall.
• Add option to check-links script. We can now specify if only internal links or internal and external links should be checked.

In addition: The CI-Pipeline is adopted, so that we check all links (internal and external) when we merge preview to main. In all other cases, just internal links will be checked.