Update top-level navigation labels

- Change 'Docs' to 'App'
- Change 'OpenHands (Core) API' to 'REST API'
- Keep 'SDK' and 'Success Stories' unchanged

Co-authored-by: openhands <openhands@all-hands.dev>

Author: openhands-agent

docs.json

@@ -20,7 +20,7 @@
   "navigation": {
     "tabs": [
       {
-        "tab": "Docs",
+        "tab": "App",
         "pages": [
           "index",
           "openhands/usage/quick-start",
@@ -261,7 +261,7 @@
         ]
       },
       {
-          "tab": "OpenHands (Core) API",
+          "tab": "REST API",
           "openapi": "openapi/openapi.json"
       }
     ],