doc: add external links, test existing links
It's possible to define extra links in the RTD theme. Those links look nice and can enhance the usability of the ARM. An example is poliastro project (make sure to view latest).
We could add similar ones to gitlab, issues, mailing list, kea homepage and support.
Also, there's a way to automatically test if external links in the sphinx doc are not broken: https://sublime-and-sphinx-guide.readthedocs.io/en/latest/references.html#test-external-links This sounds cool. We could adopt this.