update document segment (#950)

Co-authored-by: jyong <jyong@dify.ai>

api/core/docstore/dataset_docstore.py

                     content=doc.page_content,
                     word_count=len(doc.page_content),
                     tokens=tokens,
+                    enabled=False,
                     created_by=self._user_id,
                 )
                 if 'answer' in doc.metadata and doc.metadata['answer']:

api/core/indexing_runner.py

                 DocumentSegment.status == "indexing"
             ).update({
                 DocumentSegment.status: "completed",
+                DocumentSegment.enabled: True,
                 DocumentSegment.completed_at: datetime.datetime.utcnow()
             })