[Issue pixelsdb#1297] add retina docs (pixelsdb#1303)

Author: bianhq

pixels-retina/README.md

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+# Pixels Retina
+
+Retina (<ins>re</ins>al-<ins>ti</ins>me a<ins>na</ins>lytics) is the real-time data synchronization framework in Pixels.
+It supports replaying data-change operations from a log-based CDC (change-data-capture) source as mirror transactions on the columnar table data.
+It proposes a light-weight MVCC mechanism and corresponding version storage to support parallel mirror transaction replay and
+concurrent analytical query processing, and a vectorized-filter-on-read (VFoR) approach for analytical queries to read consistent
+data snapshots.
+
+Compared to the merge-on-read (MoR) based on catalog snapshots in existing lakehouse systems, 
+such as Apache Iceberg and Apache Paimon, Retina supports real row-granular (instead of batch-granular) transactional
+data change replay without the expensive version merging and data compaction mechanisms.
+Evaluations show that Retina simultaneously provides 10-ms-level data freshness and over 3.2M row/s scalable data-change 
+replay throughput, without compromising query performance or resource cost-efficiency, 
+significantly outperforming state-of-the-art lakehouses, Iceberg and Paimon, which provides minute-level data freshness
+and one order of magnitude lower data-change throughput.
+
+## Retina Components
+
+The components related to Retina are:
+
+- Sink: It connects to CDC streams from Debezium, reconstructs the data-change messages in the
+CDC stream into mirror transactions, and send the data-change operations in mirror transaction through stream RPC to Pixels-Retina.
+[Source code](https://github.com/pixelsdb/pixels-sink);
+- Replayer: It receives the data-change operations from Pixels-Sink and replays them on the columnar data tables.
+Source code: 
+[core data structures and operations](../cpp/pixels-retina), 
+[top-level replay and garbage collection](.), 
+[client](../pixels-common/src/main/java/io/pixelsdb/pixels/common/retina),
+and [server](../pixels-daemon/src/main/java/io/pixelsdb/pixels/daemon/retina).
+RPC handling are in this directory and .
+- Transaction Service: It allocates transaction timestamps for mirror transactions and analytical queries, and manages the timestamp watermarks
+for the MVCC protocol of Retina.
+Source code: 
+[client](../pixels-common/src/main/java/io/pixelsdb/pixels/common/transaction) and
+[server](../pixels-daemon/src/main/java/io/pixelsdb/pixels/daemon/transaction).
+- Index Service: It is a multi-version index that mapping the index key (e.g., primary key or secondary key) to row location. 
+Replayer looks up and updates the primary index during data-change replay.
+Source code: 
+[framework and interfaces](../pixels-common/src/main/java/io/pixelsdb/pixels/common/index),
+[pluggable implementations](../pixels-index), 
+[clients](../pixels-common/src/main/java/io/pixelsdb/pixels/common/index/service)
+[server](../pixels-daemon/src/main/java/io/pixelsdb/pixels/daemon/index)
+- Catalog Service (i.e., metadata service): It manages the schema, statistics, and data catalog of tables.
+Source code: [client](../pixels-common/src/main/java/io/pixelsdb/pixels/common/metadata) and
+[server](../pixels-daemon/src/main/java/io/pixelsdb/pixels/daemon/metadata).
+- Columnar file format: It provides the file format definition, reader, writer of the Pixels file format. [Source code](../pixels-core).
+- Trino connector: It runs inside the Trino cluster to access the services of Retina/Pixels, and calls the file reader to read data.
+[Source code](https://github.com/pixelsdb/pixels-trino).
+
+## Usage