Tutorial: Document PageXML export

Fill https://gitlab.teklia.com/arkindex/doc/-/blob/master/content/tutorial/export.md?ref_type=heads

Document:

Export format (PageXML)
folder selection
process creation
add and select the Export worker
run the process
wait for completion
look in Artefacts
Explain that the archive should be extracted using 7-zip (since it's TAR+GZ)