Docs: mark extras classes as such

> @mrdoob : I was thinking. In the list of pages, how about adding a ¹ to the items that are not included in the build. (Like BabylonLoader ¹)

I wonder about how to do this.

- Should `examples/js` classes be differentiated from `extras/` stuff ?
- Should we use kind-of-footnotes like this http://daringfireball.net/2005/07/footnotes ?
- Should these footnotes be generated in javascript ? If so, how about a new `[note:]` macro ?