Document Sass mixins

[francisrupert]

fsa-style makes uses of custom mixins, functions, etc. We could document so folks can better understand what they are, how fsa-style components use them, and how they might also do so.

I don't know that we need to do something like SassDoc (at least for an initial take on it). Also not sure how it'd fit within the system's current docs; is it a section (e.g. "Guide"), is a section on a current page, is it completely separate, etc?

Limited examples currently exist, e.g. Grid: Sass Mixins