Lift deployment README material into dedicated section

We have a very brief overview of our Google deployments in the guidebook but it misses a number of details.

In addition we have a number of post- and pre-deployment manual bootstrapping steps which are ending up scattered across READMEs. (See, e.g., the README changes in https://gitlab.developers.cam.ac.uk/uis/devops/raven/deploy-raven-core/-/merge_requests/10/)

Update the guidebook with a dedicated section on deployment. Move the existing GCP deployment overview note into the section and add:

• Manual steps required for our deployments including:
  • DNS configuration
  • Stackdriver workspace creation
  • GitLab token creation
• Information on how our webapps get deployed including:
  • CI templates
  • Deployment from staging to production via the GitLab "environments" tab.

As part of this we may want to expand visibility of the boilerplate deployment from "internal" to "public".