Updating Documentation Around GalaxyDataModule

[mkurzner]

After a brief discussion with @mwalmsley about using ZooBot we realized that some of the documentation around the GalaxyDataModule was unclear with respect to not needing to split loaded in catalog data into train and test manually as was done in the example notebook. I am opening this issue here to make note of this and to also create a thread for anyone else who runs into similar questions or issues when running ZooBot on their own data.