Add custom error file to handle unmatched URLs

### Context
Resources: [Next.js: not-found.js](https://nextjs.org/docs/app/api-reference/file-conventions/not-found)

Currently we're using the `page` router's `_error.js` to handle unmatched URLs in our application. In the `app` router, we can use `not-found.js` to handle a 404 HTTP status.

We'll need to create a `note-found.js` for handling 404.

### Problem or idea
As stated above, create a custom not-found error file to handle the application's mismatched URLs.

### Solution or next step
1. Create a `not-found.js` file to handle 404 in the `app` folder
2. Import `shared/Error/Custom404` into the file.