doc: Add a `rotate` default screenshot

lib/wibox/container/rotate.lua

@@ -1,6 +1,7 @@
 ---------------------------------------------------------------------------
 -- A container rotating the conained widget by 90 degrees.
 --
+--@DOC_wibox_container_defaults_rotate_EXAMPLE@
 -- @author Uli Schlachter
 -- @copyright 2010 Uli Schlachter
 -- @release @AWESOME_VERSION@

tests/examples/wibox/container/defaults/rotate.lua

@@ -0,0 +1,19 @@
+--DOC_HIDE_ALL
+local wibox     = require("wibox")
+
+return {
+    text   = "Before",
+    align  = "center",
+    valign = "center",
+    widget = wibox.widget.textbox,
+},
+{
+    {
+        text   = "After",
+        align  = "center",
+        valign = "center",
+        widget = wibox.widget.textbox,
+    },
+    direction = "east",
+    widget    = wibox.container.rotate
+}