docs: deprecate "category" in favor of "categories" list

MANIFEST.JSON example now uses "categories" array.
Note added: single-string "category" still supported but deprecated.

Author: ThomasFarstrike

docs/apps/creating-apps.md

@@ -51,7 +51,7 @@ In `MANIFEST.JSON`, put:
   "long_description": "Demonstrates the simplest app.",
   "fullname": "com.micropythonos.helloworld",
   "version": "0.0.2",
-  "category": "development",
+  "categories": ["Development"],
   "activities": [
     {
       "entrypoint": "hello.py",
@@ -67,6 +67,10 @@ In `MANIFEST.JSON`, put:
 }
 ```
 
+Use `"categories"` (always a list, even for a single category). Each category string is normalized to title case automatically.
+
+The deprecated single-string `"category"` field is still supported but may be removed in a future release. Apps should migrate to `"categories"`.
+
 ### Services
 
 Apps can also declare **services** — background components that run at boot time with no user interface. Services are defined in the `"services"` array of the manifest: