code: @Nonnull
    public T create(
        @Nonnull DocumentReference documentReference, @Nonnull Map<String, Object> fields) {
      return performCreate(documentReference, fields);
    }
docs: Creates a new Document at the DocumentReference's location. It fails the write if the document
  exists.
  
  @param documentReference The DocumentReference to create.
  @param fields A map of the fields and values for the document.
  @return The instance for chaining.