Change documentation startup page

[RichardPfr]

The information currently presented on the startup page is all correct and useful but might overwhelm people that are trying to get into PyRolL. Also the description of how to install PyRolL should be further up.

The startup page should rather have the following order:

1. what can you do with PyRolL
   • maybe in the form of a short GiF or Video
   • additionally a button to try it out (link to the examples on binder)
2. How to install PyRolL
   • button with a link or a fold-out of that page
3. Getting started with PyRolL
   • button with a link or a fold-out of that page
4. ... (here the further explanations like currently can be given)

Further ideas and suggestions regarding the startuppage may be collected here.

PS: @mxstrl Is this equivalent to the order you wrote down or imagined?