[MINOR] docs: add Spark Paimon catalog to Spark connector index page (#…

…6339)

### What changes were proposed in this pull request?

add Spark Paimon catalog to Spark connector index page


### Does this PR introduce _any_ user-facing change?
no

### How was this patch tested?
just document

docs/spark-connector/spark-connector.md

@@ -11,7 +11,7 @@ The Apache Gravitino Spark connector leverages the Spark DataSourceV2 interface
 
 ## Capabilities
 
-1. Supports [Hive catalog](spark-catalog-hive.md) and [Iceberg catalog](spark-catalog-iceberg.md).
+1. Supports [Hive catalog](spark-catalog-hive.md), [Iceberg catalog](spark-catalog-iceberg.md) and [Paimon catalog](spark-catalog-paimon.md).
 2. Supports federation query.
 3. Supports most DDL and DML SQLs.