docs: add docs for carousel and config-provider (#8)

* docs: add docs for carousel and config-provider

one/docs/demo/config-provider/select.vue

@@ -0,0 +1,56 @@
+<template>
+<article>
+  <veui-button
+    style="margin-right: 8px;"
+    @click="toggle"
+  >切换</veui-button>
+  <veui-config-provider :value="value">
+    <veui-select
+      :options="options"
+      clearable
+    />
+  </veui-config-provider>
+</article>
+</template>
+
+<script>
+import { Select, ConfigProvider, Button } from 'veui'
+
+const PLACEHOLDER1 = '请选择协议1'
+const PLACEHOLDER2 = '请选择协议2'
+
+// Select 的 placeholder 会响应配置 `select.placeholder`
+
+export default {
+  components: {
+    'veui-config-provider': ConfigProvider,
+    'veui-select': Select,
+    'veui-button': Button
+  },
+  data () {
+    return {
+      value: {
+        'select.placeholder': PLACEHOLDER1
+      },
+      options: [
+        {
+          label: 'MIT',
+          value: 'mit'
+        },
+        {
+          label: 'BSD',
+          value: 'bsd'
+        }
+      ]
+    }
+  },
+  methods: {
+    toggle () {
+      let prev = this.value['select.placeholder']
+      this.value['select.placeholder'] = prev === PLACEHOLDER1
+        ? PLACEHOLDER2
+        : PLACEHOLDER1
+    }
+  }
+}
+</script>