Document functional dependencies

[chexxor]

The "Functional Dependencies" section in Type Classes doesn't have any content, so we need an issue to improve visibility of this missing content.

[chexxor]

I wonder if it's ok to use the example used in @paf31's book and rephrase his explanation to better match style expected by this doc book. I don't have knowledge enough to confidently explain it, otherwise.

Wouldn't be too much harder to find someone who does have appropriate knowledge and can draft a copy for us to use here, of course.

[JordanMartinez]

I explained FDs in my learning repo here:
https://github.com/JordanMartinez/purescript-jordans-reference/blob/latestRelease/11-Syntax/01-Basic-Syntax/src/03-TypeClasses-and-Newtypes/06-Functional-Dependencies.purs

[milesfrain]

Complimentary issue in the book: purescript-contrib/purescript-book#238