docs: 添加外部导航链接

aio/content/navigation.json

@@ -545,6 +545,16 @@
       "tooltip": "关于 Angular 中类和值的详细信息。",
       "url": "api"
     },
+    {
+      "title": "互助问答",
+      "tooltip": "github 上的中文互助问答区",
+      "url": "https://github.com/ng-docs/ng-docs.github.io/issues"
+    },
+    {
+      "title": "Material 组件库",
+      "tooltip": "Angular Material 组件库的中文文档",
+      "url": "https://material.angular.cn"
+    },
     {
       "url": "guide/change-log",
       "title": "Change Log",

aio/src/app/layout/nav-item/nav-item.component.html

@@ -1,7 +1,8 @@
 <div *ngIf="!node.children">
   <a href="{{node.url}}" [ngClass]="classes" title="{{node.tooltip}}"
-    class="vertical-menu-item">
+    class="vertical-menu-item" style="position: relative" [target]="isExternalUrl(node.url)?'_blank':'_self'">
     {{node.title}}
+    <mat-icon *ngIf="isExternalUrl(node.url)">open_in_new</mat-icon>
   </a>
 </div>
 

aio/src/app/layout/nav-item/nav-item.component.ts

@@ -47,4 +47,8 @@ export class NavItemComponent implements OnChanges {
     this.isExpanded = !this.isExpanded;
     this.setClasses();
   }
+
+  isExternalUrl(url: string): boolean {
+    return /^https?:/.test(url);
+  }
 }