Update notification channel discovery and conformance

[csarven]

Resolves issue #58 by working with the notion of notification channel:

• Adds definition for notification-channel-resource.
• Describes notification-channel-discovery via notification-channel-resource (servers can advertise link relations describedby ( server-link-resource-notification-channel ) and http://www.w3.org/ns/solid#storageDescription ( server-link-storage-notification-channel ) and notification-channel description.
• Adds notification-channel-conformance

[acoburn]

I'm +1 to merge in the current form

[csarven]

Follows https://github.com/solid/notifications-panel/blob/main/meetings/2022-08-25.md#update-notification-channel-discovery-and-conformance