Update import paths in sources docs (#570)

Author: daniil-quix

docs/connectors/sources/csv-source.md

@@ -10,7 +10,7 @@ To use a CSV Source, you need to create and instance of `CSVSource` and pass it
 
 ```python
 from quixstreams import Application
-from quixstreams.sources import CSVSource
+from quixstreams.sources.core.csv import CSVSource
 
 def main():
     app = Application()

docs/connectors/sources/kafka-source.md

@@ -10,7 +10,7 @@ To use a Kafka Replicator source, you need to create an instance of `KafkaReplic
 
 ```python
 from quixstreams import Application
-from quixstreams.sources import KafkaReplicatorSource
+from quixstreams.sources.core.kafka import KafkaReplicatorSource
 
 def main():
     app = Application()

docs/connectors/sources/quix-source.md

@@ -8,7 +8,7 @@ To use a Quix Environment source, you need to create an instance of `QuixEnviron
 
 ```python
 from quixstreams import Application
-from quixstreams.sources import QuixEnvironmentSource
+from quixstreams.sources.core.kafka import QuixEnvironmentSource
 
 def main():
     app = Application()