Create z-index table in documentation/wiki

Issues like #4120 lead me to believe we should have a table of all of the z-indices used in our components, and the component parts inside them, as part of our documentation or wiki.

This would serve as a useful reference to developers trying to ensure their custom UI bits stay at the right height. We could also run it by the Material Design folks for validation to make sure we're doing it right.

@Garbee @surma What do you think?