docs: backTop tranlation (#2244)

components/back-top/demo/basic.md

@@ -1,10 +1,18 @@
 ---
 order: 0
-title: 基本
+title: 
+  zh-CN: 基本
+  en-US: Basic
 ---
 
+## zh-CN
+
 最简单的用法。
 
+## en-US
+
+The most basic usage.
+
 ````jsx
 import { BackTop } from 'antd';
 

components/back-top/demo/custom.md

@@ -1,10 +1,19 @@
 ---
 order: 1
-title: 自定义样式
+title: 
+  zh-CN: 自定义样式
+  en-US: Custom style
 ---
 
+## zh-CN
+
 可以自定义置顶按钮的样式，限制宽高：`40px * 40px`。
 
+## en-US
+
+You can customize the style of the button, just note the size limit: no more than `40px * 40px`.
+
+
 ````jsx
 import { BackTop } from 'antd';
 

components/back-top/index.en-US.md

@@ -0,0 +1,25 @@
+---
+category: Components
+type: Other
+title: BackTop
+---
+
+`BackTop` makes it easy to go back to the top of the page.
+
+## When To Use
+
+- When the page content is very long.
+- When you need to go back to the top very frequently in order to view the contents.
+
+## API
+
+> The distance to the bottom is set to `50px` by default, which is overridable.
+
+> If you decide to use custom styles, please note the size limit: no more than `40px * 40px`.
+
+
+Property | Description | Type | Default
+-----|-----|-----|------
+visibilityHeight | the `BackTop` button will not show until the scroll height reaches this value | Number | 400
+onClick | a callback function, which can be executed when you click the button | Function | -
+

components/back-top/index.md → components/back-top/index.zh-CN.md

@@ -1,7 +1,7 @@
 ---
 category: Components
-chinese: 置顶
 type: Other
+chinese: 置顶
 english: BackTop
 ---
 
@@ -15,6 +15,7 @@ english: BackTop
 ## API
 
 > 有默认样式，距离底部 `50px`，可覆盖。
+
 > 自定义样式宽高不大于 40px * 40px。
 
 | 参数        | 说明           | 类型               | 默认值       |