nnUNet UI change: Vestigial QSpinbox changed to Qlabel

Summary:
The always disabled QSpinbox showing "extra modalities" number for inferencing is now removed. Replaced QSpinbox with Qlabel showing the correct required modalities value.
Also, updated doxygen and screenshot.
Ref: T29227

Test Plan: NA

Reviewers: kalali, O1 MITK Reviewer Group I

Reviewed By: kalali, O1 MITK Reviewer Group I

Subscribers: floca

Maniphest Tasks: T29227

Differential Revision: https://phabricator.mitk.org/D688