docs: add tea overview

docs/overview.md

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+# Tea
+
+Tea is a Greenplum extension that provides read access to Iceberg tables stored in S3 in Parquet format. Currently, Tea is supported only for Greenplum 6.
+
+## Tea Runtime Environment
+
+* Greenplum. Most of the query is executed here. Tea handles only table scans.
+* Iceberg Catalog. Tea uses the catalog solely to resolve the Iceberg metadata file in S3 from a table name. Currently, only HMS is supported.
+* S3.
+* Redis or Valkey. Used by the Samovar protocol for node coordination (Samovar is required when using External Table and optional when using Foreign Table).
+
+## Tea Usage Modes
+
+### Foreign Table
+
+**Note**. When using Foreign Table, the GPORCA optimizer is not supported (a Greenplum limitation).
+
+* the master queries HMS to find where the metadata is stored;
+* reads all Iceberg metadata from S3;
+* serializes information about all data files and delete files into a single JSON and sends it to all segments;
+* each segment parses the JSON containing all metadata and determines which portion of the data it should process.
+
+### Foreign Table + Samovar
+
+Works similarly to Foreign Table, with the following exceptions:
+* metadata is distributed from the master to segments not as JSON, but by writing to a Redis queue;
+* segments pull metadata from the queue.
+
+This avoids sending all metadata to every segment and enables dynamic load balancing.
+
+### External Table + Samovar
+
+Due to Greenplum limitations, the master does not participate in query planning.
+* a segment is selected and takes on the coordinator role;
+* it parses metadata and stores it in Redis;
+* all other segments periodically check whether metadata has appeared in Redis;
+
+The main advantage is GPORCA support. The main disadvantage is high load on Redis.
+
+### Comparison
+
+* The External Table + Samovar configuration works with GPORCA, which produces better query plans.
+* The Foreign Table configuration is convenient when you do not want to maintain an extra dependency such as Redis.
+* The Foreign Table + Samovar configuration
+    * places less load on Redis than External Table + Samovar.
+    * supports dynamic load balancing during reads, unlike Foreign Table without Samovar.