Add Debian's init change tutorials to "How do I get rid of systemd?"

[knightcole]

The Debian Wiki includes two tutorials on how to change the init system: https://wiki.debian.org/Init

I recommend either adding the guide to the first section of "How do I get rid of systemd?":

Debian Wiki: Changing the init system

Or adding two links: Debian Wiki: Changing the init system - at installation time Debian Wiki: Changing the init system - on a running system