Remove Doorbell settings

https://redmine.teklia.com/issues/7947

backend#1809 (closed) removes the Doorbell integration, so all settings under the doorbell section should be removed from the configuration docs.

The 1.6.2 upgrade notes should include a section mentioning the removal of those settings. If those settings are still defined in the YAML file, it will not cause an error, but sysadmins will want to know that they can now just remove them entirely.