Link checker is missing broken links

[bfops]

See e.g. #316 and #324

[tamaro-skaljic]

Also #323

@bfops FYI: The links no longer work if you read the documentation within the repository (GitHub or locally) instead of on the website.

Maybe the website could parse the *.md files during deployment and remove all /index.mds from links, then it could work inside the repo docs as well as on the website. But I would consider this a hack which anybody forgets until it breaks. ;)