Rework link checking

- Dockerfile: Update html-proofer-plugin from version 0.8.0 to current 1.2.1
  - The new version provides functionality, we want, e.g., specification of "ignore_urls"
- mkdocs.yaml: Adjust settings for html-proofer-plugin
  - raise error after all urls have been checked
  - specifiy urls to be ignored
  - enable validation of external urls