Clarify documentation for notification

[olebhansen]

Task is to (at a minimum) address items from feedback (the same fields/questions might be applicable for multipe endpoints, e.g. item "a"):

Mangelfull API-dokumentasjon, f.eks. https://docs.altinn.studio/notifications/reference/api/openapi/#/Orders/post_orders,

f.eks.
a) Hvilke felt er obligatoriske/valgfrie?
b) Endre fra string til fatisk innhold?, f.eks.
"organizationNumber": "9siffer",
"nationalIdentityNumber": "11siffer",
c) conditionEndpoint; URL til endepunkt som kun returnerer true/false?
d) notificationChannel": "Email" i eksempel; men hva er enum?
e) emailTemplate.contentType: "Plain"i eksempel; men hva er enum?
f) emailTemplate.contentType": "Html", hvilke tags er tillatt brukt (whitelist)?

[Ahmed-Ghanam]

I have recently updated the Notification API documentation, covering most of the points outlined in this ticket. However, additional work is needed to fully meet all the specified requirements.

To avoid merge conflicts, I will move this ticket to the backlog. Once the changes are approved, I will reopen it and complete the remaining points.