Group the links on top of the /contribute page by category

This should help people to better parse it visually and find their way in this enormous list.

This was decided during the “How to get more contributors” discussion at the 2013 summit.

Blueprint: https://tails.boum.org/blueprint/contribute_section/

Feature Branch: doc/contribute-categories

Parent Task: #6165 (closed)

Original created by @bertagaz on 6173 (Redmine)