Convert markdown tutorial to rst
This fixes the warning about the archive.softwareheritage.org link in the markdown.
09:45:50 Warning, treated as error: 09:45:50 /var/lib/jenkins/workspace/DFUSE/tests-on-diff/docs/tutorial.md:85:hardcoded link 'https://archive.softwareheritage.org/api/' could be replaced by an extlink (try using ':swh_web:`api/`' instead)