📝 Add docs of FlexBox component

Author: exah

src/box/index.mdx renamed to src/box/about.mdx

@@ -7,6 +7,8 @@ import { Box } from './index'
 
 # Box Component
 
+Most basic element and most re-usable.
+
 <Playground>
   <Box bg='#000000' fg='#ffffff' ht='200px'>
     Hello World

src/flex-box/about.mdx

@@ -0,0 +1,24 @@
+---
+name: FlexBox
+---
+
+import { Link, Playground } from 'docz'
+import { FlexBox } from './index'
+
+# FlexBox Component
+
+Like `Box` component, but extended with [flexbox](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout) specific props.
+
+<Playground>
+  <FlexBox>
+    <FlexBox.Item>
+      Item
+    </FlexBox.Item>
+    <FlexBox.Item grow>
+      Item
+    </FlexBox.Item>
+    <FlexBox.Item>
+      Item
+    </FlexBox.Item>
+  </FlexBox>
+</Playground>