docs: add config.ts example

docs/.vitepress/config.ts

@@ -1,3 +1,5 @@
+import type { UserConfig } from 'vitepress'
+
 export default {
   lang: 'en-US',
   title: 'VitePress',
@@ -41,7 +43,7 @@ export default {
       '/': getGuideSidebar()
     }
   }
-}
+} as UserConfig
 
 function getGuideSidebar() {
   return [

docs/guide/configuration.md

@@ -20,4 +20,15 @@ module.exports = {
 }
 ```
 
+If you are using TypeScript, you can use `.vitepress/config.ts` instead to get better types hint for VitePress Config.
+
+```ts
+import type { UserConfig } from 'vitepress'
+
+export default {
+  title: 'Hello VitePress',
+  description: 'Just playing around.'
+} as UserConfig
+```
+
 Check out the [Config Reference](/config/basics) for a full list of options.