Move skrub.toy_order to datasets and document it

[GaelVaroquaux]

We need to:

1. move skrub.toy_order to the datasets import (aka imported as skrub.datasets.toy_order)
2. add it to the API documentation (in https://github.com/skrub-data/skrub/blob/main/doc/reference/index.rst, in the datasets section)

[MarieSacksick]

Some precisions to help for a coming sprint.

TODO

• [ ] change imports, with this line and this line.
• [ ] adapt all the tests and doctests using toy_orders
• [ ] Add it into the API documentation here