Nicer landing page

[mkroetzsch]

The main page of the documentation should look better and be more useful.

• The logo should be used in suitably big size (different logos for light and dark mode)
• Short intro paragraph that describes Nemo in a few sentences
• A list (that will grow/change later) of illustrated features/use cases (e.g., a screenshot of the online app, or a copy of the performance figure from the KR paper)
• There should be a footer with links to the KBS group page at ICCL, and links to the main nemo github and to the nemo docs github with a call for issues (this footer would be good to have on all pages)