Generate docs from src, publish to gh-pages

Fix any broken JSDoc syntax. Unfortunately there is no way to document tuples like `{[int, int]}` for a `[width, height]` array. We could possibly fix this and make the code more readable with `{width: number, height: number}` instead of tuples.

Remove single `@fileoverview` from Rectangle.js, since it was the only file with one and was weird to have that one module show up on the docs home page.

Add a `docs` script to `package.json`, run it from `npm test`, so invalid docs will cause tests to fail (eventually we may want to start using eslint-plugin-jsdoc to catch these errors with the linter alone).

.gitignore

@@ -16,3 +16,4 @@ npm-*
 /dist
 /playground/scratch-render.js
 /playground/scratch-render.js.map
+/playground/docs