code: public JsonReader beginDocument() throws IOException {
        if (currentValue.getKey() != BEGIN_OBJECT && currentValue.getKey() != BEGIN_ARRAY) {
           throw new IOException("Expecting BEGIN_OBJECT or BEGIN_ARRAY, but found " + jsonTokenToStructuredElement(
              null));
        }
        documentType = currentValue.getKey();
        consume();
        return this;
     }
docs: Begins the document
  
  @return This structured writer
  @throws IOException Something went wrong reading