The last tutorial in the "registration-to-template" section ("MTR Computation") doesn't actually use the template at all! Instead, it involves using `sct_register_multimodal` to coregister two *non-template* images together.

Additionally, in the next commits, I will be adding a second tutorial that *also* involes coregistering two non-template images together.

So, given that we will have a pair of similar tutorials, it makes sense to break them off into their own section.

…tion explanation

…omed repo

…ription