Add an example of how to write a user-in-the-loop pipeline

Summary:
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

Reviewers: max

Reviewed By: max

Subscribers: catherinewu

Differential Revision: https://dagster.phacility.com/D6561