Add new pictures to readme and docs homepage

[visr]

          I will miss those pictures, it has a nice concrete feel to it. Maybe we need a new (QGIS based?) picture that shows what you can do with Ribasim.

Originally posted by @evetion in #586 (review)

Indeed a QGIS screenshot would be nice. I prefer to have a real model (not too big), with a map behind it.

The referenced issue only removed the images from the readme, but they should also be removed/replaced in the homepage of the docs.