Add custom error file to handle unmatched URLs

[nozomione]

Context

Resources: Next.js: not-found.js

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.