Gets the parent section for the given `section`.
The parent section is the section that contains this `section`. A special case are sections of type `GTK_CSS_SECTION_DOCUMEN`T. Their parent
will either be `NULL` if they are the original CSS document that was loaded by [method@Gtk.CssProvider.load_from_file] or a section of type
`GTK_CSS_SECTION_IMPORT` if it was loaded with an `
import` rule from a different file.
the parent section