Documentation - add 'Hello World' page

[Markus-Goetz]

Feature functionality Add a page in the documentation showing a small 'Hello World' usage example.

[ClaudiaComito]

@Markus-Goetz this is covered by our tutorial, correct?

[Markus-Goetz]

More or less, it would be good to make a copy of that in the documentation as well. Moreover, it would probably good to use the split tensor figure from the paper instead of the current one

[github-actions[bot]]

Branch docs/612-Documentation_-_add_Hello_World_page created!

[mrfh92]

Meanwhile we have "Getting Started" on our README and also "TL;DR: Quick Start" (if still necessary, this can be tracked within #1186)

Therefore, closed within #1109