rename tmpdir.rst to tmp_path.rst

but also with a permanent 308 redirect from tmpdir to tmp_path so the link juice is conserved


currently the tmp_path docs are hosted at: https://docs.pytest.org/en/6.2.x/tmpdir.html#the-tmp-path-fixture

but really it would be better to use https://docs.pytest.org/en/6.2.x/tmp_path.html#the-tmp-path-fixture
