Harmonise DOIs to preferred resolver

[katrinleinweber]

Hello :-)

The DOI Foundation started recommending a new resolver. While their URL change may be a bit ironic, it's now encrypted and the old dx subdomain is being redirected. So, there is no urgency here.

However, for consistency, this PRs suggests to update all static DOI links accordingly.

Cheers!

[hagenw]

Thanks.

[katrinleinweber]

You're welcome, and thanks for merging :-)