Categorize the tutorials

Currently, [the tutorials](https://precice.org/tutorials.html) are unstructured on the website.

It's a topnav item that leads to a sidebar containing all tutorials. This looks a bit odd and is also inconsistent with `Docs` and `Community`:

  

Docs and community have titles above the sidebar, while Tutorials doesn't.
Also, there is only one pretty long folder containing all tutorials.

It may be an idea to categorize the tutorials into subfolders to split this list into logical parts and make the growing list of tutorials easier to digest.
Example:

* Introduction
* FSI
* CHT
* Fluid-Fluid
* ...