Add Quickstart page

[nilssass]

As the number of classes is increasing, it will be very hard for users to see where to begin, especially in our documentation.

I would suggest to add one documentation page after Install called Quick Guide where we briefly describe the usecases connected to each class in a meaningful hierarchy and give some super easy example. Maybe there is even a way to make this page downloadable or add a download button that will download the connected user guide as pdf

[Hendrik1704]

A first draft for the quickstart guide was implemented in #275. After the next updates of the development branch we should continue with this.