Link checker exception

Create an exception for link checker to skip links in doc.zih.tu-dresden.de/README.md.

Reason: https://gitlab.hrz.tu-chemnitz.de/zih/hpcsupport/hpc-compendium/-/blob/preview/doc.zih.tu-dresden.de/README.md#L246 link cannot be accessed without GitLab-login (404) and therefore the pipeline fails.