Add function mapped_as_dataclass

Add function mapped_as_dataclass

Added new decorator :func:`_orm.mapped_as_dataclass`, which is a function
based form of :meth:`_orm.registry.mapped_as_dataclass`; the method form
:meth:`_orm.registry.mapped_as_dataclass` does not seem to be correctly
recognized within the scope of :pep:`681` in recent mypy versions.

The new function is tested and mentioned in the docs, in 2.1 in a
subsequent patch (probably the one that adds unmapped_dataclass also)
we'll switch this new decorator to be the prominent one.

also alphabetize mapping_api.rst.  while the summary box at the top
auto-sorts, have the sidebar alpha also, it's kind of weird how
these were in no order at all

Fixes: #12855
Change-Id: If98724fd466004ec4c8a312a0cbf1c934a6ce9e3