Update the Transkribus import docs

After frontend#1020 (closed), backend#1303 (closed) and tasks#158 (closed), the Transkribus import docs should be updated to begin with the last section, then tell how to upload the exported ZIP file and import it using the file import.

This may need to mention the maximum size limit on file uploads of 2 GiB, as Transkribus collection exports can easily reach that limit.