Hello Nextcloud
I wish you and your loved ones a very happy new year!
Since the latest release of Nextcloud 28, some changes were made that caused one of the tutorials to break. The tutorial about creating a files plugin was no longer useful for a beginner. To make a files plugin, now requires for example Vue.JS but for the first tutorial we wanted to have something more simple that does not require compiling yet. So we made a new tutorial for beginners!
This tutorial will guide you through the steps to make a simple Hello World app.
Super late reply, but just FYI for anyone else who might be reading this: the skeleton app is based on GitHub - nextcloud/app_template: A template to get started with Nextcloud app development, but with the example values replaced with the form values. Yes, you could just clone the app_template repository and manually replace all the values yourself too, but that method is a bit more error-prone.