Test links in CI

[jbarnoud]

In order to avoid situations like #221 where the destination of a link was missing, it would be good to use CI to check if all the links are valid.

[orbeckst]

If we use [name]({{ site.baseurl }}{% link .... %} for internal links then the targets get automatically validated.

EDIT: or {% post_url ... %} for blog posts.

[orbeckst]

The liquid tags do not cover the external link problem mentioned above, though.

https://clementbm.github.io/github%20action/jekyll/link%20checking/2020/05/31/automatically-validate-links-on-jekyll-website.html might do the job.