Deploy public documentation to `docs.raven`

The documentation is already deployed to https://ucam-raven.readthedocs.io/. Read the docs supports custom domain hosting. Perform a similar trick to that we currently do with the guidebook:

1. Add a CNAME record to the DNS zone for production Raven so that docs.raven-868a.gcloud.automation.uis.cam.ac.uk is redirected to readthedocs.io. This should be done via terraform. It is OK for there to be corresponding docs.{...} records in the staging projects.
2. Add a vbox record to ipregister database for the docs.raven-868a.gcloud.automation.uis.cam.ac.uk host.
3. Add a CNAME from docs.raven.cam.ac.uk to docs.raven-868a.gcloud.automation.uis.cam.ac.uk in the ipregister database.
4. Configure Readthedocs to host the Raven docs from https://docs.raven.cam.ac.uk/