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 [[email protected]_from_file] or a section
of type `GTK_CSS_SECTION_IMPORT` if it was loaded with an `import` rule from a different file.
| this |
the section |
|
the parent section |