feat(website): add css for demo container (#1675)

Author: hustcc

site/pages/index.less

@@ -0,0 +1,3 @@
+div[class^='PlayGround-module--exampleContainerWrapper'] {
+  overflow: hidden;
+}

site/pages/index.zh.tsx

@@ -4,6 +4,7 @@ import { useTranslation } from 'react-i18next';
 import Banner from '@antv/gatsby-theme-antv/site/components/Banner';
 import Features from '@antv/gatsby-theme-antv/site/components/Features';
 import Cases from '@antv/gatsby-theme-antv/site/components/Cases';
+import './index.less';
 
 const IndexPage = () => {
   const { t, i18n } = useTranslation();