List Sections. Ordered by id ascending by default (pass sort_order=desc for
descending), consistent with the pagination cursor, so following the
Pagination-Next header returns every section exactly once.

The list includes the course's root section - a structural container
(parent_id: null, matching the course's root_section_id) that is not
shown in the course builder UI; most integrations can ignore it and treat its
children as the modules. Pass sort_property=sort_order for the builder's
display order; note sort_order is scoped per parent, so it is not globally
unique across the list (the root and each parent's first child both start at
0, tie-broken by id).