Make kubectl explain a useful resource for our CRDs

[edwardecook]

We don't currently support kubectl explain for any of the CRDs we have in projects-operator. The method for doing so is specifying a structural scheme, see https://kubernetes.io/docs/tasks/access-kubernetes-api/custom-resources/custom-resource-definitions/#specifying-a-structural-schema.

The initial switch to a structural schema would come with #62. However in order to gain a useful output from kubectl explain we would also need to include useful descriptions (via comments above our types and fields) that describe the purpose of the CRDs as well as the spec fields.