Use yaml or yml consistently.

[adamltyson]

In the code and the docs we have a mix of the two which is confusing. We should decide on one and stick to it (and then update the docs and brainmapper).

Image.sc issue for reference: https://forum.image.sc/t/yaml-files-for-network-retraining/104031
Docs issue: brainglobe/brainglobe.github.io#255