Add documentation for notification policies

[ClearlyClaire]

No description provided.

[trwnh]

Looks good overall but IDK if there should be a sample request under every #### Response header.

[ClearlyClaire]

Updated to remove the redundant HTTP requests examples and add missing documentation for the new entities.

[mcclure]

Can we use the presence/absence of the /api/v1/notifications/policy endpoint to detect the presence of this feature?

[ClearlyClaire]

Can we use the presence/absence of the /api/v1/notifications/policy endpoint to detect the presence of this feature?

I think so, yes.