If you'd like a "back to top" button to appear alongside the section headers in articles, just add the following code to your user Javascript or your common Javascript:

$( function () {
	var elems = document.getElementsByClassName('editsection'); // *** NOTE: ClassName may need to be 'mw-editsection' instead
	for (i = 0; i < elems.length; i  ) {
		var span = document.createElement('span');
		var link = document.createElement('a');
		link.href = '#top';
		link.appendChild(document.createTextNode('back to top'));
		span.appendChild(document.createTextNode('['));
		span.appendChild(link);
		span.appendChild(document.createTextNode('] '));
		elems[i].insertBefore(span, elems[i].firstChild);
	}
});

This only works if the user is logged in and can edit the page.