add thumbnail

Author: ammedd

docs/conf.py

@@ -76,4 +76,5 @@
     "user-guide/assignments/Research_Proposal_only": "user-guide/_images/MFP.jpg",
     "user-guide/assignments/Virtualship_research_proposal": "user-guide/_images/AnnaWeber.jpeg",
     "user-guide/assignments/sciencecommunication_assignment": "user-guide/_images/marine_ss.jpg",
+    "user-guide/tutorials/Sail_the_ship": "user-guide/_images/vessel.jpg",
 }