Add documentation for include substitution defaults (#3370)

Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>

guides/configuration-types.rst

@@ -472,6 +472,8 @@ variables can be provided to them.  This means that packages can be
 used as `templates`, allowing complex or repetitive configurations to
 be stored in a package file and then incorporated into the
 configuration more than once.
+Additionally packages could contain a ``defaults`` block which provides
+subsitutions for variables not provided by the ``!include`` block.
 
 As an example, if the configuration needed to support three garage
 doors using the ``gpio`` switch platform and the ``time_based`` cover
@@ -502,11 +504,17 @@ platform, it could be constructed like this:
           door_location: right
           open_switch_gpio: 15
           close_switch_gpio: 18
+          open_duration: "1min"
+          close_duration: "50s"
 
 
 .. code-block:: yaml
 
     # In garage-door.yaml
+    defaults:
+      open_duration: "2.1min"
+      close_duration: "2min"
+
     switch:
       - id: open_${door_location}_door_switch
         name: ${door_name} Garage Door Open Switch
@@ -524,11 +532,11 @@ platform, it could be constructed like this:
 
         open_action:
           - switch.turn_on: open_${door_location}_door_switch
-        open_duration: 2.1min
+        open_duration: ${open_duration}
 
         close_action:
           - switch.turn_on: close_${door_location}_door_switch
-        close_duration: 2min
+        close_duration: ${close_duration}
 
         stop_action:
           - switch.turn_off: open_${door_location}_door_switch