chore(docs): fix back-to-top anchor in angularjs.org doc pages

Closes https://github.com/angular/angularjs.org/issues/45

docs/src/templates/index.html

@@ -364,7 +364,7 @@
 
   <footer class="footer">
     <div class="container">
-      <p class="pull-right"><a href="#">Back to top</a></p>
+      <p class="pull-right"><a back-to-top href="#">Back to top</a></p>
 
       <p>
         Super-powered by Google  ©2010-2012

docs/src/templates/js/docs.js

@@ -372,6 +372,21 @@ docsApp.directive.errorDisplay = ['$location', 'errorLinkFilter', function ($loc
 }];
 
 
+/**
+ * backToTop Directive
+ * @param  {Function} $anchorScroll
+ *
+ * @description Ensure that the browser scrolls when the anchor is clicked
+ */
+docsApp.directive.backToTop = ['$anchorScroll', function($anchorScroll) {
+  return function link(scope, element) {
+    element.on('click', function(event) {
+      scope.$apply($anchorScroll);
+    });
+  };
+}];
+
+
 docsApp.serviceFactory.angularUrls = function($document) {
   var urls = {};