Revise documentation on MITK's top-level directories
Summary:
Revise documentation on MITK's top-level directories.
Fixes T27818.
Test Plan: None - no change in behavior.
Reviewers: O3 MITK Reviewer Group II, kalali
Reviewed By: O3 MITK Reviewer Group II, kalali
Subscribers: kalali
Maniphest Tasks: T27818
Differential Revision: https://phabricator.mitk.org/D425