fix(readingorder): assign FURNITURE content_layer to footer/header in container groups (#3044)

Elements labeled page_footer or page_header inside container groups
(e.g. key_value_region) were retaining content_layer=BODY because
_add_child_elements() did not check the element label for FURNITURE
assignment. This caused them to appear in markdown/HTML exports
despite being recognized as page furniture.

Apply the same content_layer check that _handle_text_element() uses
for standalone text elements.

Fixes #3015

Signed-off-by: g97iulio1609 <giulio97.leone@gmail.com>