feat: publicize doc implemetation

one/docs/demo/overlay/position.vue

@@ -0,0 +1,67 @@
+<template>
+<article>
+  <veui-button
+    ref="toggle"
+    @click="open = !open"
+  >
+    Toggle
+  </veui-button>
+  <veui-overlay
+    :open.sync="open"
+    overlay-class="centered-overlay"
+  >
+    <div v-outside:toggle="hide">
+      Centered
+    </div>
+  </veui-overlay>
+</article>
+</template>
+
+<script>
+import { Overlay, Button } from 'veui'
+import outside from 'veui/directives/outside'
+
+export default {
+  components: {
+    'veui-overlay': Overlay,
+    'veui-button': Button
+  },
+  directives: { outside },
+  data () {
+    return {
+      open: false
+    }
+  },
+  methods: {
+    hide () {
+      this.open = false
+    }
+  }
+}
+</script>
+
+<style lang="less">
+.centered-overlay {
+  position: fixed;
+  top: 50%;
+  left: 50%;
+  transform: translate(-50%, -50%);
+  width: 200px;
+  height: 100px;
+}
+</style>
+
+<style lang="less" docs>
+@import "~veui-theme-dls/lib.less";
+
+.centered-overlay {
+  line-height: 100px;
+  text-align: center;
+  border: 1px solid @veui-gray-color-5;
+  background-color: #fff;
+}
+</style>
+
+<docs>
+使用常见的 CSS 定位方法，可以自定义。
+</docs>