add an admonition to the rpc usage page

docs/reference/rpc.mdx

@@ -7,6 +7,12 @@ sidebar_custom_props:
     label: RPC Usage
 ---
 
+:::danger These are not the droids you're looking for
+
+This page has been migrated to the Stellar Developers documentation. Please [click here](https://developers.stellar.org/network/soroban-rpc/admin-guide) for the most up-to-date information
+
+:::
+
 The RPC service allows you to communicate directly with Soroban via a [JSON RPC interface](https://soroban.stellar.org/api/).
 
 For example, you can build an application and have it [send a transaction](https://soroban.stellar.org/api/methods/sendTransaction), [get ledger](https://soroban.stellar.org/api/methods/getLedgerEntries) and [event data](https://soroban.stellar.org/api/methods/getEvents), or [simulate transactions](https://soroban.stellar.org/api/methods/simulateTransaction).