Notes from the 2025 lectures

[GabrieleDiCorato]

This pull request is to propose minor, mainly aesthetic, updates. The material is slightly expanded with notes and solutions taken during the 2025 edition of the course. Changes include:

1. Updated all links to the python documentation from python 2 to 3;
2. Fixed section numbering (some lessons started from "1.1" instead of "1");
3. Reviewed exercises: they now have a code block for the student's solution, and an expandable markdown block showing the solution presented in class;
4. Run nb-clean on all lesson notebooks, to clean all cell execution counts, metadata, and outputs. At the moment this is not automatically enforced in this project, so this is a proposal intended to facilitate contributions;
5. Fixed typos and uniformed the notation used to denote code in markdown cells (`);
6. Performed minor rewording, mainly using ChatGPT.