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?

[LukasWallrich]

@max-fw any update on this? Would really help SEO if we get rid of broken links :)

[max-fw]

Yes, most of the broken links are due to the tags missing the https://
E.g. will erroneously render to the URL https://www.forrt.org/folder/www.example.com

Will be updating in various pages.

[LukasWallrich]

Working on this in PR #705. The new link checker uses lychee and downloads the latest build artifact to check all links (internal + external) across the full site.

As part of this, we're also converting publisher-specific DOI URLs (e.g. https://journals.sagepub.com/doi/10.1177/...) to canonical https://doi.org/... URLs. This makes them checkable — doi.org returns proper 404s for invalid DOIs, whereas publishers like Sage, T&F, and APA block all automated requests with 403 regardless of whether the link is valid.

Going forward, https://doi.org/{doi} should be the preferred format for all DOI links in the repository.