docs: add release notes

Author: UdjinM6

doc/release-notes-7101.md

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+## Indexes
+
+### Async Index Migration (#7101)
+
+`TimestampIndex`, `SpentIndex`, and `AddressIndex` have been migrated from synchronous to asynchronous operation, following the same pattern as `TxIndex` and other indexes using `BaseIndex` framework.
+
+When enabling an index for the first time, the node will build the index in the background while remaining fully operational. Progress can be monitored via the `getindexinfo` RPC.
+
+Existing nodes with indexes enabled will automatically migrate data from the old location (block index database) to new separate databases on first startup. This migration may take 20-40 minutes or longer depending on hardware specifications and index sizes. The node will log progress during migration.
+
+Breaking changes:
+- `SpentIndex` and `AddressIndex` are incompatible with pruned nodes as they require access to undo data now