Add GitHub action to check markdown files for broken links

[morri-son]

What would you like to be added:
Add GitHub action to check markdown files for broken links, e.g. Add GitHub action to check markdown files for broken links or anything else suitable. Ideally the actions should be executed on every push to main and added to the required tests on a PR, but depending on how long it runs, we may just schedule it.

Why is this needed:
To avoid dead links in any of our markdown documentation files in any of the repositories, we should add a Github action that checks for dead links to all repositories.

linked to #161

[hilmarf]

e.g. https://github.com/corona-warn-app/cwa-server/blob/main/.github/workflows/markdown-analysis.yml#L27