docs: Add documentation for [delayed_gcode]

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

config/example-extras.cfg

@@ -1693,3 +1693,16 @@
 #release_gcode:
 #   A list of G-Code commands to execute when the button is released.
 #   G-Code templates are supported.
+
+# Execute a gcode on a set delay.
+#[delayed_gcode my_delayed_gcode]
+#initial_duration: 0.
+#   The duration of the initial delay (in seconds).  If set to a non-zero
+#   value the delayed_gcode will execute the specified number of seconds
+#   after the printer enters the "ready" state.  This can be useful for
+#   initialization procedures or a repeating delayed_gcode.  If set to 0
+#   the delayed_gcode will not execute on startup.  Default is 0.
+#gcode:
+#   A list of G-Code commands to execute when the delay duration has
+#   elapsed.  G-Code templates are supported.  This parameter must be
+#   provided.