[Build] optimize webpack config

build/webpack.doc.js

@@ -0,0 +1,13 @@
+const path = require('path');
+const devConfig = require('./webpack.dev.js');
+
+module.exports = Object.assign({}, devConfig, {
+  mode: 'production',
+  output: {
+    path: path.join(__dirname, '../docs/dist'),
+    publicPath: 'https://img.yzcdn.cn/zanui/vant/',
+    filename: '[name].[hash:8].js',
+    umdNamedDefine: true,
+    chunkFilename: 'async_[name].[chunkhash:8].js'
+  }
+});