Mark App Store apps API endpoints as experimental (#21379)

API endpoints are associated with the following experimental features:
- #19447
- #18867 

Fleet's handbook
[here](https://fleetdm.com/handbook/company/product-groups#experimental-features):
![Screenshot 2024-08-16 at 1 25
33 PM](https://github.com/user-attachments/assets/e0c76309-aae3-4c2d-9685-5f046060115a)

docs/REST API/rest-api.md

@@ -9050,6 +9050,8 @@ Content-Type: application/octet-stream
 
 ### List App Store apps
 
+> **Experimental feature**. This feature is undergoing rapid improvement, which may result in breaking changes to the API or configuration surface. It is not recommended for use in automated workflows.
+
 Returns the list of Apple App Store (VPP) that can be added to the specified team. If an app is already added to the team, it's excluded from the list.
 
 `GET /api/v1/fleet/software/app_store_apps`
@@ -9098,6 +9100,8 @@ Returns the list of Apple App Store (VPP) that can be added to the specified tea
 
 ### Add App Store app
 
+> **Experimental feature**. This feature is undergoing rapid improvement, which may result in breaking changes to the API or configuration surface. It is not recommended for use in automated workflows.
+
 _Available in Fleet Premium._
 
 Add App Store (VPP) app purchased in Apple Business Manager.
@@ -9162,6 +9166,8 @@ Body: <blob>
 
 ### Install package or App Store app
 
+> **Experimental feature**. This feature is undergoing rapid improvement, which may result in breaking changes to the API or configuration surface. It is not recommended for use in automated workflows.
+
 _Available in Fleet Premium._
 
 Install software (package or App Store app) on a macOS, iOS, iPadOS, Windows, or Linux (Ubuntu) host. Software title must have a `software_package` or `app_store_app` added to be installed.