docs: fix nav and some enhancements

packages/docs/docs/.vuepress/nav/zh.js

@@ -27,7 +27,7 @@ module.exports = [
           },
           {
             text: '官方插件',
-            link: '/plugin/official.html'
+            link: '/zh/plugin/official.html'
           }
         ]
       },
@@ -57,11 +57,11 @@ module.exports = [
         items: [
           {
             text: '设计理念',
-            link: '/miscellaneous/design-concepts.html'
+            link: '/zh/miscellaneous/design-concepts.html'
           },
           {
             text: '从 0.x.x 迁移',
-            link: '/miscellaneous/migration-guide.html'
+            link: '/zh/miscellaneous/migration-guide.html'
           }
         ]
       },

packages/docs/docs/miscellaneous/design-concepts.md

@@ -193,6 +193,6 @@ With the goal of decoupling, we were able to separate VuePress into the followin
 - [@vuepress/core](https://github.com/vuejs/vuepress/tree/master/packages/@vuepress/core)：Including the core implementation of `dev`, `build` and `Plugin API`;
 - [@vuepress/theme-default](https://github.com/vuejs/vuepress/tree/master/packages/@vuepress/theme-default)：The default theme you see now.
 
-Of course, for most users, you don't need to care about the three libraries. Package [vuepress](https://www.npmjs.com/search?Q=vuepress) has already assembled the three packages together, so you can use VuePress like `0.x`.
+Of course, for most users, you don't need to care about these three libraries, package [vuepress](https://www.npmjs.com/search?Q=vuepress) has already assembled them together, so you can use VuePress like `0.x`.
 
 

packages/docs/docs/zh/miscellaneous/design-concepts.md

@@ -193,4 +193,4 @@ i18n UI 最终的路由将是 `/i18n/`.
 - [@vuepress/core](https://github.com/vuejs/vuepress/tree/master/packages/@vuepress/core)：包含 dev、build 的核心实现和 Plugin API；
 - [@vuepress/theme-default](https://github.com/vuejs/vuepress/tree/master/packages/@vuepress/theme-default)：你现在所看到的默认主题。
 
-当然，对于大多数用户来说，你并不需要关心上述三个库，[vuepress](https://www.npmjs.com/search?q=vuepress) 这个包已经将上述三个包组装在一起，你完全可以像 `0.x` 那样使用 VuePress。
+当然，对于大多数用户来说，你并不需要关心上述三个库，[vuepress](https://www.npmjs.com/search?q=vuepress) 这个包已经将上述三个包组装在一起，因此你完全可以像 `0.x` 那样使用 VuePress。