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/knowledgebase_service.py

@@ -60,6 +60,9 @@ class KnowledgebaseService(CommonService):
         if offset < 0 or offset > kbs_length:
             raise IndexError("Offset is out of the valid range.")
 
+        if count == -1:
+            return kbs[offset:]
+
         return kbs[offset:offset+count]
 
     @classmethod