List z-index values in docs

[meridius]

I think it would be great if you could show the list of used z-index numbers in the documentation on http://getbootstrap.com/

[cvrebert]

The problem would be keeping the information up-to-date. I suppose a Grunt task or npm script could do the job though.

[mdo]

Our major z-index values haven't changed really at all. We can call out the stack in the docs I think.