✨(models/api) allow inviting external users to a document by their email

We want to be able to share a document with a person even if this person
does not have an account in impress yet.

This code is ported from https://github.com/numerique-gouv/people.

src/backend/core/urls.py

@@ -20,6 +20,12 @@ document_related_router.register(
     viewsets.DocumentAccessViewSet,
     basename="document_accesses",
 )
+document_related_router.register(
+    "invitations",
+    viewsets.InvitationViewset,
+    basename="invitations",
+)
+
 
 # - Routes nested under a template
 template_related_router = DefaultRouter()