Create tutorials for SecML-Torch

[maurapintor]

Create tutorials for the library. Good points to start this are:

• The examples folder https://github.com/pralab/secml-torch/tree/main/examples
• The tutorials for SecML https://github.com/pralab/secml/tree/master/tutorials

Tutorials should be well documented (see SecML, including references to the papers) and self-contained (do not require connecting one tutorial with the other).

Additionally, it's recommended to use Colab notebooks.

Feel free to post below some ideas for the tutorials as comments.