Add GKE runner initial documentation

Initially I've refactored the platform related sections into a Cloud Platform reference section and renamed the remaining deployment section to App Deployment. I feel that this provides a clearer separation between core platform related reference and boilerplate/app deployment referenace material.

Secondly, I've added a reference section and a couple of initial how-tos for the GKE runner setup. I'm sure this will require additional effort going forward but I think this is at least a good start to build from.

Closes #190 (closed)