[python/square-root] Add mentoring doc

[antoniouaa]

Hello, my attempt at mentoring notes for square-root for python. I see there's already been a past attempt at introducing this mentoring document , in #2251.

Document includes sections suggesting and explaining 2 different solutions, main solution using Heron's method from the wiki pages linked in the description of the problem. I understand that's what the problem statement is pushing people to investigate and implement, instead of using cheap hacks like ** 0.5 and pow.

Comments welcome!