Update text and pngs of the point set interaction documentation

Summary:
Mainly updated and extended the description of the single buttons and
replaced the images

Fixes T27103

Test Plan: website validation

Reviewers: O3 MITK Reviewer Group II, kislinsk

Reviewed By: O3 MITK Reviewer Group II, kislinsk

Subscribers: kompan, kislinsk

Maniphest Tasks: T27103

Differential Revision: https://phabricator.mitk.org/D368