Add an example of how to write a user-in-the-loop pipeline
This diff adds an example of how to write pipelines to include a user in the loop.
The documentation is added.
The description for some examples has a period at the end, others hasn't. To be consistent, I also modified those description.
Test Plan: See tests
Reviewed By: max
Differential Revision: https://dagster.phacility.com/D6561