Add section about functionality classes to CSS & JS.

[fleeting]

As discussed with @smhigley and @defvayne23, need to work out a sass file that contains functionality classes that are required for JavaScript components to work correctly. No preference on .js-dropdown or .do-dropdown as the format of said classes (leaning towards .do-).

These classes would be used to trigger things in JavaScript and contain the minimum CSS required for it to be functional. These classes should never contain other styling like color, font-size, borders, etc. Typically it would just be translate, position, display, transition, etc.