Consider adding extrepo to Debian install instructions

[AudunVN]

In the Linux desktop install instructions for Debian and Ubuntu at

docs/usage/install-on-desktops.md

Lines 68 to 95 in c072d04

	## Ubuntu/Debian and derivatives
	For Droidian, skip directly to the last step
	For Ubuntu Touch, these steps are not needed
	Make sure you have Wayland Session enabled (Ubuntu 22.04+)
	{% embed url="https://linuxconfig.org/how-to-enable-disable-wayland-on-ubuntu-22-04-desktop" %}
	* Install pre-requisites
	```bash
	sudo apt install curl ca-certificates -y
	```
	* Add the official repository
	```bash
	curl -s https://repo.waydro.id | sudo bash
	```
	If the script fails to detect your distribution, you can provide a valid option by appending `-s <DISTRO>`.
	Currently supported values are: **mantic**, **focal**, **jammy**, **kinetic**, **lunar**, **noble**, **plucky**, **questing**, **bookworm**, **bullseye**, **trixie**, **sid**
	* Install waydroid
	```bash
	sudo apt install waydroid -y
	```
	Then start Waydroid from the applications menu.

the docs currently instruct the user to download and run a shell script as root to add the Waydroid repo.

Making users install things this way is... less than ideal from a security perspective (in general, not saying that the shell script at https://repo.waydro.id nor Waydroid are in themselves insecure). The main Debian-y solution to avoid users doing it this way these days is via extrepo, which has had the Waydroid repo in it for two years. This seems to be working well, installed Waydroid on Debian 13 using it today:

# if you don't have extrepo installed: sudo apt update && sudo apt install extrepo
sudo extrepo enable waydroid
sudo apt update
sudo apt install waydroid

Not sure whether there are any edge cases which the shell script handle that extrepo doesn't; I suppose the instructions with the shell script and/or manually adding the repo and its keys could still be useful for people running older versions of Debian-based distros that don't have extrepo.