Document conditional package !include (#5536)

* doc conditional packages

* formatting

* Update content/components/packages.md

Co-authored-by: Jonathan Swoboda <154711427+swoboda1337@users.noreply.github.com>

---------

Co-authored-by: Jonathan Swoboda <154711427+swoboda1337@users.noreply.github.com>

Author: jpeletier

content/components/packages.md

@@ -168,6 +168,29 @@ switch:
     # ...
 ```
 
+## Conditionally including a package
+
+You can include a package based on a condition, or choose a package dinamically by loading your package with
+`!include` into a substitution variable instead of under `packages`:
+
+```yaml
+# In config.yaml
+substitutions:
+  left_garage_door: !include
+    file: garage-door.yaml
+    vars:
+      door_name: Left
+
+  enable_extra_door: true
+
+packages:
+  extra_door: ${ left_garage_door if enable_extra_door else {} }
+```
+
+The above utilizes a [Jinja expression](/components/substitutions#jinja-expressions) to determine whether
+`left_garage_door` package is actually included. `enable_extra_door` can be set from the
+[command line](/components/substitutions#command-line-substitutions) as well.
+
 {{< anchor "config-packages_extend" >}}
 
 ## Extend