Out-of-memory Example/Explanation/Documentation

I am linking to a question on [SO](https://stackoverflow.com/questions/66305852/out-of-memory-operations-in-python-datatable-package-how-to-do-it), regarding out-of-memory datasets. I dont have the proper understanding to answer, and would also love to learn more about this. Tagging @st-pasha @oleksiyskononenko.

[SO link](https://stackoverflow.com/questions/66305852/out-of-memory-operations-in-python-datatable-package-how-to-do-it)