functions.phpへ
add_action('init','add_categories_for_pages');
function add_categories_for_pages(){
register_taxonomy_for_object_type('category', 'page');
}
add_action( 'pre_get_posts', 'nobita_merge_page_categories_at_category_archive' );
function nobita_merge_page_categories_at_category_archive( $query ) {
if ( $query->is_category== true && $query->is_main_query() ) {
$query->set('post_type', array( 'post', 'page', 'nav_menu_item'));
}
}
を追加する
