doc: Add a `margin` default screenshot

lib/wibox/container/margin.lua

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

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

@@ -0,0 +1,54 @@
+--DOC_HIDE_ALL
+local wibox     = require("wibox")
+local beautiful = require("beautiful")
+
+return {
+    nil,
+    {
+        nil,
+        {
+            {
+                text   = "Before",
+                align  = "center",
+                valign = "center",
+                widget = wibox.widget.textbox,
+            },
+            bg       = beautiful.bg_highlight,
+            widget   = wibox.container.background
+        },
+        nil,
+        expand = "none",
+        layout = wibox.layout.align.horizontal,
+    },
+    nil,
+    expand = "none",
+    layout = wibox.layout.align.vertical,
+},
+{
+    nil,
+    {
+        nil,
+        {
+            {
+                {
+                    text   = "After",
+                    align  = "center",
+                    valign = "center",
+                    widget = wibox.widget.textbox,
+                },
+                bg       = beautiful.bg_highlight,
+                widget   = wibox.container.background
+            },
+            top    = 5,
+            left   = 20,
+            color  = "#ff0000",
+            widget = wibox.container.margin,
+        },
+        nil,
+        expand = "none",
+        layout = wibox.layout.align.horizontal,
+    },
+    nil,
+    expand = "none",
+    layout = wibox.layout.align.vertical,
+}