[Improvement] NavBar: add zIndex prop (#525)

docs/markdown/en-US/nav-bar.md

@@ -52,6 +52,7 @@ export default {
 | right-text | Right Text | `String` | `''` | - |
 | left-arrow | Whether to show left arrow | `Boolean` | `false` | - |
 | fixed | Whether to fixed top | `Boolean` | `false` | - |
+| z-index | Z-index | `Number` | `1` | - |
 
 ### Slot
 

docs/markdown/zh-CN/nav-bar.md

@@ -53,6 +53,7 @@ export default {
 | right-text | 右侧文案 | `String` | `''` | - |
 | left-arrow | 是否显示左侧箭头 | `Boolean` | `false` | - |
 | fixed | 是否固定在顶部 | `Boolean` | `false` | - |
+| z-index | 元素 z-index | `Number` | `1` | - |
 
 ### Slot