doc: Add a flex layout default screenshot

lib/wibox/layout/flex.lua

@@ -1,4 +1,6 @@
 ---------------------------------------------------------------------------
+--
+--@DOC_wibox_layout_defaults_flex_EXAMPLE@
 -- @author Uli Schlachter
 -- @copyright 2010 Uli Schlachter
 -- @release @AWESOME_VERSION@

tests/examples/wibox/layout/defaults/flex.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.flex.horizontal
+}