Check hyperlinks automatically

[MLopez-Ibanez]

It shouldn't be too hard to check that hyperlinks work in the bibtex files in one of the github actions. This will detect some typos and also stale links.

[MLopez-Ibanez]

To make it less expensive and less noisy, we should probably just check the links added or modified by a commit. Otherwise, the action would check thousands of links for every commit.

[MLopez-Ibanez]

The new checkurl can be used for this. Before adding it to github actions, we would need to fix all the broken URLs. There are quite a few.

[MLopez-Ibanez]

• Fix URLs in biblio.bib or add broken ones to bad-urls.txt
• Fix URLs in articles.bib or add broken ones to bad-urls.txt
• Call checkurl from github action (maybe a separate one)