Document how to use save_dataset to overwrite existing files

[hyanwong]

When playing with sgkit, I seem to be wanting to save the sgkit dataset multiple times. If I have already saved the dataset to the same location, it complains that "path '' contains a group", because the default writing mode is "w-". Could it be documented that to overwrite existing file you need to specify

sg.save_dataset(ds, "test.zarr", mode="w")

[tomwhite]

#1125 and linked issues may be related too.