add-ons docs talk about using Codenotary CAS, but that seems to be a dead service / proprietary product offering

I can't find any live information on the internet about CAS; all the links seem to be dead or timeout.
Is this really a current thing that is used by home assistant?

Please update the docs:

* If CAS is still a thing that can be used, update with current information on how to notarize
* If CAS is no longer a thing, update to remove references to it

