docs(nav): highlight current nav-index-listing

Color the current nav-index-listing item dark red
to make it easier to know where you are.

Closes #9970
Closes #9974

docs/app/assets/css/docs.css

@@ -128,6 +128,10 @@ h1,h2,h3,h4,h5,h6 {
   margin-bottom:5px;
 }
 
+.nav-index-group .nav-index-listing.current a {
+  color: #B52E31;
+}
+
 .nav-breadcrumb {
   margin:4px 0;
   padding:0;

docs/app/src/docs.js

@@ -13,6 +13,7 @@ angular.module('DocsController', [])
   $scope.navClass = function(navItem) {
     return {
       active: navItem.href && this.currentPage && this.currentPage.path,
+      current: this.currentPage && this.currentPage.path === navItem.href,
       'nav-index-section': navItem.type === 'section'
     };
   };