how to warp existing annotations to a new template, using `brainglobe-registration`

Depends on https://github.com/brainglobe/brainglobe-napari-io/issues/118

This should take the form of a "How-to" guide in [the diataxis sense](https://diataxis.fr/how-to-guides/).

It should include the required prerequisites:
* a python environment with `brainglobe-registration` installed.
* the new template
* the desired annotations as a starting point for the new template

And the steps will include~
* finding the registration map between the old template (with the desired annotations) and the new template
* applying the registration map to the annotations
* exporting the annotations (to nifti if you want to use ITK snap for editing them)