API: created list_doc (#1327)

### What problem does this PR solve?

Adds the api of listing documentation.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

api/db/services/document_service.py

@@ -59,6 +59,35 @@ class DocumentService(CommonService):
 
         return list(docs.dicts()), count
 
+    @classmethod
+    @DB.connection_context()
+    def list_documents_in_dataset(cls, dataset_id, offset, count, order_by, descend, keywords):
+        if keywords:
+            docs = cls.model.select().where(
+                (cls.model.kb_id == dataset_id),
+                (fn.LOWER(cls.model.name).contains(keywords.lower()))
+            )
+        else:
+            docs = cls.model.select().where(cls.model.kb_id == dataset_id)
+
+        total = docs.count()
+
+        if descend == 'True':
+            docs = docs.order_by(cls.model.getter_by(order_by).desc())
+        if descend == 'False':
+            docs = docs.order_by(cls.model.getter_by(order_by).asc())
+
+        docs = list(docs.dicts())
+        docs_length = len(docs)
+
+        if offset < 0 or offset > docs_length:
+            raise IndexError("Offset is out of the valid range.")
+
+        if count == -1:
+            return docs[offset:], total
+
+        return docs[offset:offset + count], total
+
     @classmethod
     @DB.connection_context()
     def insert(cls, doc):