Update docs to use Raspberry Pi Imager

[Sawtaytoes]

I'd recommend using the official Raspberry Pi Imager app in your docs instead of Rufus. In the GUI, you can install the correct version of Alpine without having to manually download it, and the imager is free of viruses (Rufus in an older build I had once) and spyware (Etcher). It's also got a CLI installer for Linux.