docs: update external links (#1969)

packages/docs/docs/.vuepress/components/Bit.vue

@@ -3,6 +3,7 @@
     <a
       href="https://www.bitsrc.io/?utm_source=vue&utm_medium=vue&utm_campaign=vue&utm_term=vue&utm_content=vue"
       target="_blank"
+      rel="noopener noreferrer"
     >
       <span>This project is sponsored by</span>
       <img

packages/vuepress/README.md

 <p align="center">
-  <a href="https://vuepress.vuejs.org/" target="_blank">
+  <a href="https://vuepress.vuejs.org/" target="_blank" rel="noopener noreferrer">
     <img width="180" src="https://raw.githubusercontent.com/vuejs/vuepress/master/packages/docs/docs/.vuepress/public/hero.png" alt="logo">
   </a>
 </p>