Agree on and document Terraform coding preferences

# 💡 Summary #

We should come to consensus on various Terraform-related coding/naming conventions and document them.

## Motivation and context ##

In https://github.com/cisagov/cool-assessment-terraform/pull/153 there was some discussion about how to best name Terraform resources that can contain one or multiple instances of that resource. We decided in that PR to pluralize the name of those resources and their related descriptions. If that is our decision, we should document it somewhere and abide by it.

## Implementation notes ##

None

## Acceptance criteria ##

- [ ] Terraform-related coding and/or naming conventions have been documented
- [ ] The above conventions can be applied to our code reviews going forward