Improve Debian section, substitute link to the old wiki with a link to wiki.debian.org

https://wiki.mobian.org/doku.php?id=desktopenvironments is the old and deprecated wiki. To avoid confusion, I've quickly set-up some install instructions to the official Debian wiki. Mobian project is using the Debian wiki for all their pages, as well.

Edit: install instructions will likely change soon, as the plasma-mobile-* metapackages have just been accepted into Debian. After the situation has cleared and settled, I will update the Debian wiki. Anyway, no need to point users to the old Mobian wiki...