Describe the process of setting up a custom 404 'Not Found' page in Next.js.

Creating a custom 404 'Not Found' page in Next.js is straightforward. You simply need to create a file named `404.js` inside your `pages` directory (i.e., `pages/404.js`). When a user navigates to a non-existent URL, Next.js will automatically render this `404.js` component. This page is statically generated at build time, meaning it's highly performant. You can customize its content, styling, and even add links back to your homepage or other relevant sections of your site.