Signpost the old ably gem's API reference from the new docs front page

ably.com/docs links only the newest package's API reference, so users
still on the maintenance-only ably gem land here; the front-page notice
points them at the frozen old reference and the migration guide.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: umair-ably

INTRO.md

@@ -1,5 +1,10 @@
 # Ably `Ruby` Client Library SDK API Reference
 
+> **Using the `ably` gem?** This is the API reference for the `ably-pubsub-server` gem, which replaces it.
+> The API reference for the maintenance-only `ably` gem remains available at
+> [sdk.ably.com/builds/ably/ably-ruby/main/docs](https://sdk.ably.com/builds/ably/ably-ruby/main/docs/)
+> until its end of life, and the [migration guide](https://github.com/ably/ably-pubsub-ruby/blob/main/UPDATING.md) covers moving over.
+
 The `Ruby` Client Library SDK supports a realtime and a REST interface.
 
 The realtime interface enables a client to maintain a persistent connection to Ably and publish, subscribe and be present on channels.