Add App.getInitialProps example to the TypeScript page (#13785)
Add App.getInitialProps example to the TypeScript page (#13785)

I added the commented code with the `AppContext` type to match the example from the [Custom App](https://nextjs.org/docs/advanced-features/custom-app) page. But since this is the TS page I think we could remove these comments and use a clean example, what do you think? 🤔
zeit/next.js,flybayer/next.js,azukaru/next.js,zeit/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,flybayer/next.js,JeromeFitz/next.js,azukaru/next.js,flybayer/next.js,zeit/next.js,azukaru/next.js,JeromeFitz/next.js,JeromeFitz/next.js