docs(config): document CUBEJS_DB_TYPE

Author: hassankhan

docs/content/Configuration/Environment-Variables-Reference.mdx

@@ -525,6 +525,14 @@ If `true`, enables SSL encryption for database connections from Cube.
 | --------------- | ---------------------- | --------------------- |
 | `true`, `false` | N/A                    | N/A                   |
 
+## `CUBEJS_DB_TYPE`
+
+A database from the list of [supported databases][ref-config-db].
+
+| Possible Values                                       | Default in Development | Default in Production |
+| ----------------------------------------------------- | ---------------------- | --------------------- |
+| [Any valid database supported by Cube][ref-config-db] | N/A                    | N/A                   |
+
 ## `CUBEJS_DB_URL`
 
 The URL for a database.