Protection against None when sending notifications

[stsewd]

I remember we put this logic outside the task, so we don't create additional tasks. But we also refactor the send_notifications method to receive a pk instead of using self.version because if the API fails, it will be None.

ref https://sentry.io/organizations/read-the-docs/issues/1425845969/?project=148442&query=is%3Aunresolved

[stale]

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[ericholscher]

I think sending emails on PR builds makes sense, but I can understand it might be spammy. I think we likely need to make things a bit more configurable.

I'm 👍 on this with an update to our docs explaining when we send which notifications.