Callbacks and Webhooks, with examples

[lornajane]

We get quite a few questions about callbacks and webhooks, the use cases and the differences. Add some resources to make these topics clearer.

[lornajane]

Do I need to mention @SensibleWood to make them a collaborator before I can assign?

[SensibleWood]

Grabbing as next most useful thing I can crank out a page or two for