Adds Math operations and Variables to python-turtle workshop

[fitoria]

As part of the workshop given on PyLatam last month, the need for a intro to variables on the python turtle workshop was seen, I've added a page to introduce students to variables and simple math operations that can be used as part of the python-turtle workshops to make it easier to understand how to draw polygons and integrate that into functions.

I've placed it right after: "Activity 4 - Simplifying code with functions"

Source branch: https://github.com/fitoria/workshops/tree/fitoria/turtle-variables-update

file: https://github.com/fitoria/workshops/blob/8b8eb71e4f92a04ab00748368a9d3ff63a3c3ddd/content/english/python-turtle/math-and-variables.md