Docs: ReasonReact API

[peterpme]

A lot of folks have been asking about a documented ReasonReact API

• mentioning stuff that's going away / deprecated
• going through the API and at a high level, talking about each function.

There are curried versions of that function, but we should talk about them under each top level, for example:

• useEffect talk about useEffect0, useEffect1, etc, here
• useState

[yawaramin]

I've been thinking of adding some API docs to ReasonReact. I can take a shot at this.

[davesnx]

Closing this since it's duplicated https://github.com/reasonml/reason-react/issues/473