add mkinitcpio hook for Arch and sample /etc/mkinitcpio.conf

[DerVerruckteFuchs]

This adds the necessary initcpio hook for Arch to boot from an encrypted root bcachefs drive/partition. For the time being, the hook is kept in the custom hooks directory (/etc/initcpio/hooks) as per the Arch wiki. As things get mainlined and become part of the kernel, we may want to move the hook into the system hooks directory (/usr/lib/initcpio/hooks). There is an example of a working /etc/mkinitcpio.conf that calls the bcachefs hook when creating a new initramfs with mkinitcpio -p linux-bcachefs.