Move Reference Documentation "Preparations" to separate notebook

Currently, the data for the Reference Documentation is retrieved in the fifth notebook: https://stag.training.fdz.ioer.info/notebooks/305_mapping.html#creating-maps

I added this code when I could not start this notebook.

I think this download script should be in notebook 300_preparation.ipynb, so participants can go sequentially through notebooks.

The alternative is to add it to every notebook in the beginning (but a waste of space).

At some stage, there should also be a step (e.g. in the end) that clears all data in 00_data, or maybe at the beginning of 300_preparation.ipynb, as a means to "reset" the directory and start from scratch. This would guarantee that the notebooks are 100% reproducible, as any manually edited or renamed file in 00_data would be reset/deleted.