update docs for KinesisFirehoseEvent

Author: ryandeivert

docs/utilities/data_classes.md

@@ -76,6 +76,7 @@ Event Source | Data_class
 [DynamoDB streams](#dynamodb-streams) | `DynamoDBStreamEvent`, `DynamoDBRecordEventName`
 [EventBridge](#eventbridge) | `EventBridgeEvent`
 [Kafka](#kafka) | `KafkaEvent`
+[Kinesis Firehose Delivery Stream](#kinesis-firehose-delivery-stream) | `KinesisFirehoseEvent`
 [Kinesis Data Stream](#kinesis-streams) | `KinesisStreamEvent`
 [Lambda Function URL](#lambda-function-url) | `LambdaFunctionUrlEvent`
 [Rabbit MQ](#rabbit-mq) | `RabbitMQEvent`
@@ -892,6 +893,45 @@ or plain text, depending on the original payload.
         do_something_with(data)
     ```
 
+### Kinesis Firehose Delivery Stream
+
+Kinesis Firehose Data Transformation can use a Lambda Function to modify the records
+inline, and re-emit them back to the Delivery Stream.
+
+Similar to Kinesis Data Streams, the events contain base64 encoded data. You can use the helper
+function to access the data either as json or plain text, depending on the original payload.
+
+=== "app.py"
+
+    ```python
+    import base64
+    import json
+    from aws_lambda_powertools.utilities.data_classes import event_source, KinesisFirehoseEvent
+
+    @event_source(data_class=KinesisFirehoseEvent)
+    def lambda_handler(event: KinesisFirehoseEvent, context):
+        result = []
+        for rec in event.records:
+            # if data was delivered as json; caches loaded value
+            data = kinesis_firehose_record.data_as_json
+            
+            # or swap for below if data was delivered as text
+            # data = kinesis_firehose_record.data_as_text
+            
+            modified_record = do_sometime(data)
+
+            firehose_record_output = {
+                "recordId": rec.record_id,
+                "data": base64.b64encode(json.dump(modified_record).encode('utf-8')),
+                "result": "Ok"
+            }
+
+            result.append(firehose_record_output)
+        
+        # return transformed records
+        return = {'records': result}
+    ```
+
 ### Lambda Function URL
 
 === "app.py"