docs: mention `id` field in `1-abstract-dto.rst`

[sobolevn]

It caught my attention that it was never explained: why do we have an extra id field in the response. Since this part is dedicated to explaining how request / response fields are marked, I think that it is important to mention that id is special:

I had to grep what .my_lib is and how class _Base(CommonTableAttributes, UUIDPrimaryKey, DeclarativeBase): is constructed. Only finding UUIDPrimaryKey and its id: Mapped[UUID] = mapped_column(default=uuid4, primary_key=True) has anwsered my question :)