doc: Add a ratio layout default screenshot

lib/wibox/layout/ratio.lua

@@ -2,6 +2,8 @@
 --- A layout filling all the available space. Each widget is assigned a
 -- ratio (percentage) of the total space. Multiple methods are available to
 -- ajust this ratio.
+--
+--@DOC_wibox_layout_defaults_ratio_EXAMPLE@
 -- @author Emmanuel Lepage Vallee
 -- @copyright 2016 Emmanuel Lepage Vallee
 -- @release @AWESOME_VERSION@

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

@@ -0,0 +1,13 @@
+local generic_widget = ... --DOC_HIDE
+local wibox     = require("wibox") --DOC_HIDE
+
+local w = wibox.widget {
+    generic_widget( "first"  ),
+    generic_widget( "second" ),
+    generic_widget( "third"  ),
+    layout  = wibox.layout.ratio.horizontal
+}
+ 
+w:ajust_ratio(2, 0.44, 0.33, 0.22)
+
+return w --DOC_HIDE