Exploring the API tutorial

Some additional comments:
* once 3.0.0 is released, it would probably be better to change this tutorial to load the build/css for that rather than master
* vectors could be added, as they're quite a bit different from tile/image layers - or this could be in a separate tutorial
* I've not included changes to the index, but would suggest a review of documentation and tutorials. I would get rid of the current concepts tutorial, which should be covered by this and by the API docs themselves