nnUNnet documentation & screenshots update
Summary: Ref: T29134 - Documentation update for changes after hackathon.
Test Plan: NA
Reviewers: kislinsk, kalali, O1 MITK Reviewer Group I, floca
Reviewed By: O1 MITK Reviewer Group I, floca
Subscribers: floca
Differential Revision: https://phabricator.mitk.org/D628