Afficher les catégories enfants dans une page taxonomy (#849)

assets/sass/_theme/design-system/taxonomies.sass

assets/sass/_theme/sections/categories.sass

assets/sass/_theme/sections/posts.sass

assets/sass/_theme/utils/sidebar.sass

hugo.yaml

layouts/events_categories/term.html

layouts/organizations_categories/term.html

layouts/partials/categories/categories.html

layouts/partials/categories/category.html

layouts/partials/taxonomies/categories.html

layouts/persons_categories/term.html

layouts/posts_categories/term.html

layouts/projects_categories/term.html