This project is obsolete for now (Pillar 8. Febr 2020).
Archetypes are templates for Pillar projects.
Each subdirectory becomes available as a project skeleton to the pillar archetype command, e.g. pillar archetype book should give you a fresh template of a book project, ready to build and to write.
To add a new archetype, create a new folder and put your files there. When you modify any of the files of the available archetype, run the ci https://ci.inria.fr/pharo-contribution/job/Pillar-Archetypes/. It will zip the files and the command can use them.
To know more about archetypes read https://ci.inria.fr/pharo-contribution/job/EnterprisePharoBook/lastSuccessfulBuild/artifact/book-result/PillarChap/Pillar.html#archetypes