docs: Add a changelog entry for emqx_persistent_session_ds

changes/ce/feat-12089.en.md

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+Added a technical preview of the new persistent session implementation based on RocksDB.
+Please note that this feature is in alpha stage and must not be enabled in the production systems.
+
+Features missing in the early preview version of the new persistent session implementation:
+
+- Shard failover
+- Retained messages
+- Will message handling
+- Shared subscriptions
+- Subscription IDs