add documentation of terraform deployments

IMPORTANT This MR is incomplete but already contains a great deal of documentation. I'm putting it up for review for comment on the currently completed sections to aid the review load.

This is a WIP containing a basic skeleton for documentation on deployments and content for a few pages within it. I think that filling out the rest of the pages should end up with a pretty comprehensive documentation of how we deploy things.

Viewable at https://guidebook.devops.uis.cam.ac.uk/en/issue-64-deployment/deployment/ while not merged.

Closes #64 (closed)