Added documentation on activateSPFeature (SharePoint#4444)

Added documentation on activateSPFeature as there are quite some misunderstandings around it not working for site scoped features due to lack of documentation. Trying to take that away with this update.

Author: KoenZomers

docs/declarative-customization/site-design-json-schema.md

@@ -856,6 +856,28 @@ Use the **associateExtension** action to register a deployed SharePoint Framewor
 }
 ```
 
+## Activate a Feature
+
+Use the **activateSPFeature** action to activate a web scoped feature.
+
+> [!NOTE]
+> Site scoped features cannot be activated through Site Designs at this time.
+
+#### JSON values
+
+- **featureId** – The ID of the web scoped feature to activate. 
+
+#### Example
+
+To enable the web scoped feature which allows for Events Lists to be created (feature ID 00bfea71-ec85-4903-972d-ebe475780106):
+
+```json
+{
+    "verb": "activateSPFeature",
+    "featureId": "00bfea71-ec85-4903-972d-ebe475780106"
+}
+```
+
 ## Trigger a flow
 
 Use the **triggerFlow** verb to kick off a custom flow.