Tutorial: Mention Transkribus support

Add a new section on https://doc.arkindex.org/tutorial/corpus. This short section will talk about uploading PageXML files to Arkindex. Two options:

• Transkribus Collections exports are supported (give link to https://doc.arkindex.org/howto/transkribus/),
• Some PageXML files are supported using our CLI tool https://cli.arkindex.org/upload/#page-xml-documents (advanced)