Replace screenshots of 'introduction' in the docs and readme's

[kaskranenburgQ]

With the starting page redesign we should closely look where this is used in the documentation an update these where necessary.

The issue etmodel/4602 blocks this issue, since we first need to finalise the design before we can update the pictures.

[kaskranenburgQ]

In my first inspection of the documentation, the file interface.png is the only picture that I encountered that needs updating conform the introduction update.
However, the pictures in the rest of the screenshots in the User manual section are also desperately in need of an update.
This might call for a bigger update of the screenshots in the docs.

[kaskranenburgQ]

In this issue we should also fix the old link in the documentation as pointed out by @kndehaan in this comment.