docs: add korean translation site link (#805)

* add korean translation site

* change  domain to v3.ko.vuejs.org

Author: narusas

src/.vuepress/config.js

@@ -409,6 +409,10 @@ module.exports = {
         text: 'Translations',
         link: '#',
         items: [
+          {
+            text: '한국어',
+            link: 'https://v3.ko.vuejs.org/'
+          },
           {
             text: '中文',
             link: 'https://v3.cn.vuejs.org/'