[exponent-docs] Document how to use custom icon fonts
[exponent-docs] Document how to use custom icon fonts

Added a few paragraphs on custom icon font usage with Exponent. Currently, the information needed to understand how this works is located in three different places, so I tried to sum it all up in one generic example for people to get an overview of how it works.

This was my first time using  markup, so hopefully I didn'\''t mess anything up 🙏

Cheers!

Closes #13

exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent