docs: support nav config for attr of external links

Author: billyyyyy3320

Diff for: packages/docs/docs/theme/default-theme-config.md

@@ -54,6 +54,19 @@ module.exports = {
 }
 ```
 
+Outbound links automatically get `target="_blank" rel="noopener noreferrer"`. You can offer `target` and `rel` to customize the attributes:
+
+``` js
+// .vuepress/config.js
+module.exports = {
+  themeConfig: {
+    nav: [
+      { text: 'External', link: 'https://google.com', target:'_self', rel:'' },
+    ]
+  }
+}
+```
+
 These links can also be dropdown menus if you provide an array of `items` instead of a `link`:
 
 ```js

Diff for: packages/docs/docs/zh/theme/default-theme-config.md

@@ -48,6 +48,20 @@ module.exports = {
     ]
   }
 }
+
+
+```
+外部链接 `<a>` 标签的特性将默认包含`target="_blank" rel="noopener noreferrer"`，你可以提供 `target` 与 `rel`，它们将被作为特性被增加到 `<a>` 标签上：
+
+``` js
+// .vuepress/config.js
+module.exports = {
+  themeConfig: {
+    nav: [
+      { text: 'External', link: 'https://google.com', target:'_self', rel:'' },
+    ]
+  }
+}
 ```
 
 当你提供了一个 `items` 数组而不是一个单一的 `link` 时，它将显示为一个 `下拉列表` ：