Fix link checker & broken (external) URLs

[LukasWallrich]

We have a link check action, but it does not work (only finds 1 URL to check).
https://github.com/forrtproject/forrtproject.github.io/actions/workflows/link-check.yaml

Maybe we rather need our own simple crawler script? @justinsulik thanks for taking a look

[github-actions]

🤖 Hi @LukasWallrich, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.

[LukasWallrich]

@justinsulik any update on this?