taxonomies: merge tags and category taxonomies

(see https://gohugo.io/content-management/taxonomies/)

We currently list categories inside some menus and manually build the URL to point to the corresponding tag. This is a dirty hack that should be removed.

For example: http://localhost:1313/fr/challenges/reverse/ != http://localhost:1313/fr/reverse/

Removing this hack would also allow setting descriptions on categories page.