Lecture Review: Restructuring Advanced Python Programming Section, Task 1

[HumphreyYang]

Hi @jstac and @mmcky,

This is part 1 of a series of PR on #210.

This part addresses the first task:

Move the section on Recursive Function to 4. Function, mark it as advanced content, and add an exercise based on the factorial function in exercise 1

Could you please kindly review the change and let me know if there is anything I need to change?

[jstac]

Nice work @HumphreyYang , many thanks.

@mmcky , please merge if you are also happy.

[mmcky]

thanks @HumphreyYang