Regenerate webhook for read the docs

[sroet]

@dwhswenson I assume you got the same emails as me, but I don't have the permissions to fix it. (don't have admin rights on this repo or on RtD)

Our current RtD hook does not seems to use a secret, which will be required from January 31! You should click on the 3 links, and "resync webhooks" which should update the secret. (couldn't test it though, if I try it I get the error: Webhook activation failed. Make sure you have the necessary permissions.)