docs/client-linux-wireguard.md: please add how to setup though the integrated Ubuntu GUI

https://github.com/trailofbits/algo/blob/master/docs/client-linux-wireguard.md

![image](https://github.com/user-attachments/assets/0274161c-5415-4f6b-876d-bcaf7efbc192)

![image](https://github.com/user-attachments/assets/def59e74-366e-4ac2-a505-59ca6bd8cdb3)
