docs: refactor the navbar, add entry for v4->v5 migration guide

Author: haoqunjiang

docs/.vuepress/config.js

@@ -116,12 +116,22 @@ module.exports = {
             ]
           },
           {
-            text: 'Migrating From v3',
-            link: '/migrating-from-v3/'
-          },
-          {
-            text: 'Changelog',
-            link: 'https://github.com/vuejs/vue-cli/blob/dev/CHANGELOG.md'
+            text: 'Migrate from Older Versions',
+            items:
+            [
+              {
+                text: 'From Vue CLI v3 to v4',
+                link: '/migrations/migrate-from-v3/'
+              },
+              {
+                text: 'From Vue CLI v4 to v5',
+                link: '/migrations/migrate-from-v4/'
+              },
+              {
+                text: 'Full Changelog',
+                link: 'https://github.com/vuejs/vue-cli/blob/dev/CHANGELOG.md'
+              }
+            ]
           }
         ],
         sidebar: {

docs/migrating-from-v3/README.md renamed to docs/migrations/migrate-from-v3.md

@@ -2,7 +2,7 @@
 sidebar: auto
 ---
 
-# Migrating from v3
+# Migrate from v3
 
 First, install the latest Vue CLI globally:
 

docs/migrations/migrate-from-v4.md

@@ -0,0 +1,28 @@
+---
+sidebar: auto
+---
+
+# Migrate from v4
+
+First, install the latest Vue CLI globally:
+
+```sh
+npm install -g @vue/cli
+# OR
+yarn global add @vue/cli
+```
+
+## Upgrade All Plugins at Once
+
+In your existing projects, run:
+
+```sh
+vue upgrade
+```
+
+And then see the following section for detailed breaking changes introduced in each package.
+
+------
+
+## One-By-One Manual Migration
+