docs: add versions

Author: posva

docs/.vuepress/config.js

@@ -92,6 +92,12 @@ module.exports = ctx => ({
             text: 'API Reference',
             link: '/api/'
           },
+          {
+            text: 'v3.x',
+            items: [
+              { text: 'v4.x', link: 'https://next.router.vuejs.org' }
+            ]
+          },
           {
             text: 'Release Notes',
             link: 'https://github.com/vuejs/vue-router/releases'
@@ -142,6 +148,12 @@ module.exports = ctx => ({
             text: 'API 参考',
             link: '/zh/api/'
           },
+          {
+            text: 'v3.x',
+            items: [
+              { text: 'v4.x', link: 'https://next.router.vuejs.org/zh/' }
+            ]
+          },
           {
             text: '更新记录',
             link: 'https://github.com/vuejs/vue-router/releases'
@@ -192,6 +204,12 @@ module.exports = ctx => ({
             text: 'API リファレンス',
             link: '/ja/api/'
           },
+          {
+            text: 'v3.x',
+            items: [
+              { text: 'v4.x', link: 'https://next.router.vuejs.org' }
+            ]
+          },
           {
             text: 'リリースノート',
             link: 'https://github.com/vuejs/vue-router/releases'
@@ -241,6 +259,12 @@ module.exports = ctx => ({
             text: 'Справочник API',
             link: '/ru/api/'
           },
+          {
+            text: 'v3.x',
+            items: [
+              { text: 'v4.x', link: 'https://next.router.vuejs.org' }
+            ]
+          },
           {
             text: 'История изменений',
             link: 'https://github.com/vuejs/vue-router/releases'
@@ -291,6 +315,12 @@ module.exports = ctx => ({
             text: 'API 레퍼런스',
             link: '/kr/api/'
           },
+          {
+            text: 'v3.x',
+            items: [
+              { text: 'v4.x', link: 'https://next.router.vuejs.org' }
+            ]
+          },
           {
             text: '릴리즈 노트',
             link: 'https://github.com/vuejs/vue-router/releases'
@@ -340,6 +370,12 @@ module.exports = ctx => ({
             text: 'API',
             link: '/fr/api/'
           },
+          {
+            text: 'v3.x',
+            items: [
+              { text: 'v4.x', link: 'https://next.router.vuejs.org' }
+            ]
+          },
           {
             text: 'Notes de version',
             link: 'https://github.com/vuejs/vue-router/releases'