[Build] revert site to webpack config (#3747)

docs/site/desktop/main.js

@@ -0,0 +1,41 @@
+import Vue from 'vue';
+import VueRouter from 'vue-router';
+import VantDoc from '@vant/doc';
+import App from './App';
+import routes from '../router';
+import { isMobile, importAll } from '../utils';
+
+if (isMobile) {
+  location.replace('mobile.html' + location.hash);
+}
+
+Vue.use(VueRouter).use(VantDoc);
+
+const docs = {};
+const docsFromMarkdown = require.context('../../markdown', false, /(en-US|zh-CN)\.md$/);
+const docsFromPackages = require.context('../../../src', true, /README(\.zh-CN)?\.md$/);
+
+importAll(docs, docsFromMarkdown);
+importAll(docs, docsFromPackages);
+
+const router = new VueRouter({
+  mode: 'hash',
+  routes: routes({ componentMap: docs })
+});
+
+router.afterEach(() => {
+  window.scrollTo(0, 0);
+  Vue.nextTick(() => window.syncPath());
+});
+
+window.vueRouter = router;
+
+if (process.env.NODE_ENV !== 'production') {
+  Vue.config.productionTip = false;
+}
+
+new Vue({
+  el: '#app',
+  render: h => h(App),
+  router
+});