Add download info to top of intro_yaxarrays notebook

[felixcremer]

We should add the description how to download the data that we use in the YAXArrays intro notebook to the top of the notebook so that people can follow along.
The info is currently in the exercise YAXArrays slides, but there is nothing wrong in duplicating that info

using YAXArrays
using Zarr
era5url = "https://s3.bgc-jena.mpg.de:9000/deepextremes/v3/ERA5Cube.zarr"
ds = open_dataset(era5url)
sub = ds[Ti=DateTime(1998,1,1)..DateTime(2022,12,31), longitude=0..14.76,latitude=30.1..60]
savedataset(sub,path="./era5.zarr")