Explains how to make an actor transparent so that other actors
are visible through it.
Also explains a bit more generally about opacity and how
it's computed from the actor, container, and color; and how actor
visibility is affected by depth (fog) and depth order.
Use a modified version of the Poky Handbook CSS for the HTML version of
the Cookbook.
Promote Elliot as author.
Re-license from the GPLv2.0 to the CC BY-NC-SA 2.0.
