Docs: Add note to Quickstart about quickstart app not being in INSTALLED_APPS

[Jagan6991]

Hello,

In the Quickstart tutorial, the quickstart app is created but never added to the INSTALLED_APPS list in settings.py.

While this works for the tutorial (because the quickstart app has no models, admin, or templates), it can be confusing for beginners who are learning best practices. In any real-world project, failing to add an app to INSTALLED_APPS will cause errors with migrations, templates, and the admin panel.

I suggest adding a small note to clarify this, something like:

Note: We don't add the 'quickstart' app to INSTALLED_APPS here because it doesn't contain any models. In a real project, you should always add your new app to INSTALLED_APPS immediately so Django can find its models, templates, and admin files.