build: add files of vant-cli 2

packages/vant-cli/site/mobile/components/DemoSection.vue

@@ -0,0 +1,32 @@
+<template>
+  <section class="van-doc-demo-section" :class="demoName">
+    <slot />
+  </section>
+</template>
+
+<script>
+import decamelize from 'decamelize';
+
+export default {
+  name: 'demo-section',
+
+  computed: {
+    demoName() {
+      const { meta } = this.$route;
+      if (meta && meta.name) {
+        return `demo-${decamelize(meta.name, '-')}`;
+      }
+
+      return '';
+    }
+  }
+};
+</script>
+
+<style lang="less">
+.van-doc-demo-section {
+  box-sizing: border-box;
+  min-height: 100vh;
+  padding-bottom: 20px;
+}
+</style>