Hi There,
That is how it works when fixed sidebar is used. It is build that way because there’s not enough space below the menu. You have extra space because you have five menu links only. Usually when using more than that all space will be used. It is not recommended to add more content below it. In case you insist, we can add using javascript. Please add this on Appearance > Customize > Custom Javascript:
jQuery(function($){
$( "<h1>Title</h1><p>Content goes here</p>" ).insertAfter( $( ".x-nav-wrap.desktop" ) );
});
Hope this helps.