Tutorial: Corpus import from ZIP archive of images

This should be the promoted way to go. ZIP all images of the Pellet dataset and create a public download link.

tutorial/corpus/#import-data-to-arkindex should explain how to upload that ZIP to Arkindex.

The current explanations (with the scraper) should be moved to a separate section at the end of the page. Add a warning at the beginning explaining that this section is intended for advanced users.