Request: add nextcloudpi to raspberry pi imager

Description

Raspberry pi imager already provides images of other third party operating systems and adding nextcloudpi would simplify installing it. This can be achieved by opening an issue in https://github.com/raspberrypi/rpi-imager with a json with the required links (see https://github.com/raspberrypi/rpi-imager/issues/162 )

Example

OctoPi json can be used as a reference:

{
    "os_list": [
        {
            "name": "OctoPi",
            "description": "A Raspberry Pi distribution for 3d printers. Ships OctoPrint out-of-the-box.",
            "icon": "https://raw.githubusercontent.com/guysoft/OctoPi/devel/media/rpi-imager-OctoPi.png",
            "subitems_url": "http://unofficialpi.org/rpi-imager/rpi-imager-octopi.json"
        }
    ]
}

Aditionally, icon needs to be provided as a 40x40 image

Start by reviewing the Raspberry Pi Imager issue 162 linked in the description and compare its JSON structure with the OctoPi example. Prepare the required links and a 40x40 icon for NextcloudPi, then open the requested issue in raspberrypi/rpi-imager; done means the upstream request includes the complete JSON and icon.