SHACL shapes for data models

[csarven]

(This is an initial process. May need to create a general one in the solid/shapes repo and/or mention in https://github.com/solid/specification/blob/main/CONTRIBUTING.md )

• Create SHACL shapes for Notifications Protocol data models to be published at (base URL): https://www.w3.org/ns/solid/shapes/notifications. Ensure that each shape refers back to the specification section describing it. SHACL shapes of each data model in Notification Channel Types can be done any time.
• Update the Notifications Protocol to refer to the shape URL.
• PR the shape document at https://github.com/solid/shapes . Refer to the specification URL and this issue.