docs: add japanese link to translations menu (vuejs#1011)

Author: kazupon

src/.vuepress/config.js

@@ -430,6 +430,10 @@ module.exports = {
             text: '한국어',
             link: 'https://v3.ko.vuejs.org/'
           },
+          {
+            text: '日本語',
+            link: 'https://v3.ja.vuejs.org/'
+          },
           {
             text: 'More Translations',
             link: '/guide/contributing/translations#community-translations'