Draft: rework adding_operator_tutorial

Context

This MR is dedicated to rework the Operator Adding tutorial.
The previous version contained the code files which was not easy to maintain. In this version we separate the tutorial in two sections:

• section I: we display pre-compiled code as an example to understand different parts to create operators.
• section II: we give a test case for the user to create a new operator and provide guidelines to do that.

As the necessary files for operator Hardmax were already coded, they will be added to the Aidge Plateform in MR
The new operator to add will be Shrink.

Fixes issue #237