doc: Add a stack layout default screenshot

lib/wibox/layout/stack.lua

@@ -8,6 +8,7 @@
 -- The indices are going from 1 (the bottom of the stack) up to the top of
 -- the stack. The order can be changed either using `:swap` or `:raise`.
 --
+--@DOC_wibox_layout_defaults_stack_EXAMPLE@
 -- @author Emmanuel Lepage Vallee
 -- @copyright 2016 Emmanuel Lepage Vallee
 -- @release @AWESOME_VERSION@

tests/examples/wibox/layout/defaults/stack.lua

@@ -0,0 +1,10 @@
+local generic_widget = ... --DOC_HIDE
+local wibox     = require("wibox") --DOC_HIDE
+
+return --DOC_HIDE
+wibox.widget {
+    generic_widget( "first"  ),
+    generic_widget( "second" ),
+    generic_widget( "third"  ),
+    layout  = wibox.layout.stack
+}