chore(docs-app): show loader when loading view / partial

Closes #14385
PR (#15280)

docs/app/src/docs.js

@@ -23,6 +23,11 @@ angular.module('DocsController', [])
   $scope.$on('$includeContentLoaded', function() {
     var pagePath = $scope.currentPage ? $scope.currentPage.path : $location.path();
     $window._gaq.push(['_trackPageview', pagePath]);
+    $scope.loading = false;
+  });
+
+  $scope.$on('$includeContentError', function() {
+    $scope.loading = false;
   });
 
   $scope.$watch(function docsPathWatch() {return $location.path(); }, function docsPathWatchAction(path) {
@@ -31,6 +36,8 @@ angular.module('DocsController', [])
 
     var currentPage = $scope.currentPage = NG_PAGES[path];
 
+    $scope.loading = true;
+
     if (currentPage) {
       $scope.partialPath = 'partials/' + path + '.html';
       $scope.currentArea = NG_NAVIGATION[currentPage.area];