docs: add migration guide for upgrading to Vite+

  - Document prerequisites (Vite 8.x and Vitest 4.x requirements)
  - Explain automatic migration via 'vite migration' command
  - Provide manual migration steps for all package managers
  - Include troubleshooting section and post-migration checklist

Author: fengmk2

docs/.vitepress/config.mts

@@ -52,6 +52,10 @@ export default defineConfig({
               text: 'Getting Started',
               link: '/guide/',
             },
+            {
+              text: 'Migration',
+              link: '/guide/migration',
+            },
           ],
         },
         {