[3.13] Doc: C API: Demote sections to subsections for consistency (GH-126535) (#126546)

Doc: C API: Demote sections to subsections for consistency (GH-126535)

The entire file should be a single section; the headings below the
first heading should be subsections.
(cherry picked from commit e3510bd3dd9ea8f2a30cb1128470aee3a48d8880)

Co-authored-by: Richard Hansen <rhansen@rhansen.org>