Add VoxelMorph tutorial

[kvttt]

Is your feature request related to a problem? Please describe.
Going off of a previous comment on the recently added VoxelMorph module. It would be helpful to create a corresponding tutorial on task 3 of Learn2Reg 2021 challenge.

Describe the solution you'd like
A tutorial using the VoxelMorph module. Feel free to assign me to this issue. :)