docs: Add CUBEJS_DB_MATERIALIZE_CLUSTER

Author: igorlukanin

docs/pages/reference/configuration/environment-variables.mdx

@@ -560,6 +560,14 @@ for pre-aggregation generation.
 | -------------------------------------------------- | ---------------------- | --------------------- |
 | Whether to force fetch columns in ordinal position | `true`                 | `true`               |
 
+## `CUBEJS_DB_MATERIALIZE_CLUSTER`
+
+The cluster name to use when connecting to [Materialize](/product/configuration/data-sources/materialize).
+
+| Possible Values                                           | Default in Development | Default in Production |
+| --------------------------------------------------------- | ---------------------- | --------------------- |
+| A valid Materialize cluster name                          | N/A                    | N/A                   |
+
 ## `CUBEJS_DB_SNOWFLAKE_ACCOUNT`
 
 The Snowflake account identifier to use when connecting to the database.