Documentation change: add protocol generator example

[becky-gilbert]

This PR moves the protocol generator documentation change from develop into master.

The documentation change is a new protocol generator example showing how to implement a custom prior participation check for the study and child at the start of a new study session.

The documentation change adds a new section label to the EFP docs that is referenced by the Lookit documentation (lookit-docs repo), and the Lookit documentation grabs the section labels from the EFP master branch. So this change needs to be merged into master order for the Lookit docs -> EFP docs link to work.