[MD Extensions] Add a details subpage, move to one list

chrome/browser/resources/md_extensions/manager.css

 /* Copyright 2015 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 :host {
   height: 100%;
 }
 
 #panel {
   --paper-header-panel-standard-container: {
     display: flex;
   };
 }
 
-#items {
+#pages {
   -webkit-margin-start: 30px;
   overflow-y: auto;
   padding-bottom: 30px;
   width: 100%;
 }
 
 extensions-item {
   display: inline-block;
 }