Kanaal documentatie url should respect configured subpath

[marnixdessing]

The register_kanalen command does not respect the configured subpath in open-zaak.

The following error is returned by open-notifications when running the command on an open-zaak instance configured with a subpath: De URL https://api.example.com/ref/kanalen/#autorisaties gaf als antwoord HTTP 404. Geef een geldige URL op. The url should be https://api.example.com/open-zaak/ref/kanalen/#autorisaties as i've configured the SUBPATH environment variable as open-zaak

See:

notifications-api-common/notifications_api_common/management/commands/register_kanalen.py

Line 35 in 7abd32e

f"{protocol}://{domain}{reverse('notifications:kanalen')}#{kanaal}"