text: Title: Support for nested lists in Docs - DocArray V2
  Body: I want to use DocArray V2 0.32 to store nested objects where each object is a list and can have its embedding. The idea is to extract multilevel categories and key phrases from a longer text and store these in a sub-index in a Qdrant. But whenever I try and set it fails to convert when sending over gRPC. 
  
  First, should DocArray support this kind of nesting or just flat sub-classes? Second, if it should support this, I'm not sure if it is a bug or if there is another way of doing this?
  
  **The error:**
  
  │ /opt/homebrew/lib/python3.11/site-packages/docarray/base_doc/mixins/io.py:312 in                 │
  │ _get_content_from_node_proto                                                                     │
  │                                                                                                  │
  │   309 │   │   │   │   return_field = getattr(value, content_key)                                 │
  │   310 │   │   │                                                                                  │
  │   311 │   │   │   elif content_key in arg_to_container.keys():                                   │
  │   312 │   │   │   │   field_type = cls.__fields__[field_name].type_ if field_name else None      │
  │   313 │   │   │   │   return_field = arg_to_container[content_key](                              │
  │   314 │   │   │   │   │   cls._get_content_from_node_proto(node, field_type=field_type)          │
  │   315 │   │   │   │   │   for node in getattr(value, content_key).data  
  KeyError: 'terms'
  
  **Below is the basic setup:**
  
  class TermDoc(BaseDoc):
      text: Optional[str]
      embedding: Optional[NdArray[dim_size]] = Field(space=similarity)
      
  class CategoryDoc(BaseDoc):
      level: int
      text: str
      embedding: Optional[NdArray[dim_size]] = Field(space=similarity)
  
  class ProductDoc(BaseDoc):
      sku: Optional[str]
      ...
      categories: Optional[List[CategoryDoc]]
      terms: Optional[List[TermDoc]]
    