Add example types to user compensation value statements description

[jstrothman]

Background The Guide's Plan page describes how we compensate participants. In September 2021, we were able to successfully provide examples of when not compensating participants blocked user research

Acceptance criteria

• [ ] https://ux-guide.18f.gov/research/plan/#financial-compensation now includes a description of the information needed in examples and a link to the google doc with examples

Definition of done

• [ ] One of the POs has approved the change