Add Debian RISC-V

[benz0li]

RISC-V is now an official Debian architecture: https://lists.debian.org/debian-riscv/2023/07/msg00053.html

The prebuilt is available at https://dl.b-data.ch/utm/debian-unstable-riscv64-utm4.zip

_virtual_machines/debian-riscv64.md:

---
title: Debian RISC-V
architecture: RISC-V64
memory: 1024 MiB
disk: 10 GiB
display: Console
spice_installed: false
username: debian
password: debian
screenshot: to be defined
download: to be defined
utm_link: true
---
## Notes
* Root account username/password is `root`/`password`. You should change this.
* This is a Debian image generated by the [Debian Quick Image Baker](https://people.debian.org/~gio/dqib/).
* Suite: unstable; APT source: `deb [arch=riscv64] http://deb.debian.org/debian/ unstable main`.