Add monitoring for build job

[GuySartorelli]

The queued job that builds the docs might break sometimes. This happened recently (see #140) where the job was stuck in a "paused" state.

We don't have any monitoring for this situation. In some cases the docs site could be partially built, which is less likely to be discovered naturally. We should add some monitoring so if the job doesn't run to completion someone on the CMS Squad is notified and can check on it.