Create a Style Guide

[jgeorges371]

There needs to be some kind of style guide for when we want to declare a definition or highlight an important part of the content of a tutorial.

There also needs to be a centralized place where contributors can go and look up and reference the style guide.

Kudos goes to @rich1126 for bringing this up.

[Johny Georges]

Here's the first iteration of the style guide.

https://github.com/JetJet13/mathbook/wiki/Tutorial-Style-Guide