Start page with general information about the system
The current start page is the exploration page. For a first-time user, it would be nice to display a page with a short introduction and links to the user-docs.
If a user is signed-in, he should be redirected to the exploration page.
Further improvements could be to make this page customizable by a system-admin (custom text/images/...), which needs #550 (closed) and #362 (closed).