UCL docs for Node.js (#2014)

@renejeglinsky @smahati this addition should be sap-internal (no
secrets, just not applicable to externals yet), but i didn't know how to
achieve this as the menu (Node → cds.Services) is also affected...

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Co-authored-by: Rene Jeglinsky <rene.jeglinsky@sap.com>

Author: sjvans

node.js/_menu.md

@@ -32,6 +32,7 @@
   ## [Class cds. MessagingService](messaging)
   ## [Class cds. DatabaseService](databases)
   ## [Serving Fiori UIs](fiori)
+  ## [Platform Services](../../node.js/platform-services)
 
 # [cds. Events](events)