TypeScript documentation for _app.tsx (vercel#10345)

* TypeScript documentation for _app.tsx

* Use relative URL

* Use relative URL

* Update docs/basic-features/typescript.md

Co-Authored-By: Luis Alvarez D. <luis@zeit.co>

* Update docs/basic-features/typescript.md

Co-Authored-By: Luis Alvarez D. <luis@zeit.co>

Co-authored-by: Luis Alvarez D. <luis@zeit.co>

docs/advanced-features/custom-app.md

@@ -10,6 +10,7 @@ Next.js uses the `App` component to initialize pages. You can override it and co
 - Keeping state when navigating pages
 - Custom error handling using `componentDidCatch`
 - Inject additional data into pages
+- [Add global CSS](/docs/basic-features/built-in-css-support#adding-a-global-stylesheet)
 
 To override the default `App`, create the file `./pages/_app.js` as shown below:
 
@@ -41,6 +42,10 @@ The `Component` prop is the active `page`, so whenever you navigate between rout
 
 > Adding a custom `getInitialProps` in your `App` will disable [Automatic Static Optimization](/docs/advanced-features/automatic-static-optimization.md).
 
+### TypeScript
+
+If you’re using TypeScript, take a look at [our TypeScript documentation](/docs/basic-features/typescript#custom-app).
+
 ## Related
 
 For more information on what to do next, we recommend the following sections:

docs/basic-features/typescript.md

@@ -118,3 +118,17 @@ export default (req: NextApiRequest, res: NextApiResponse<Data>) => {
   res.status(200).json({ name: 'John Doe' })
 }
 ```
+
+## Custom `App`
+
+If you have a [custom `App` ](/docs/advanced-features/custom-app), you can use the built-in type `AppProps`, like so:
+
+```ts
+import { AppProps } from 'next/app'
+
+function MyApp({ Component, pageProps }: AppProps) {
+  return <Component {...pageProps} />
+}
+
+export default MyApp
+```