Document Use of Cloud Build

[NimJay]

• This repo has about 10 cloudbuild.yaml files.
• They were introduced in:
  • https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/pull/184 and
  • https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/pull/185.
• We use these cloudbuild.yaml files to docker build our sample apps and push them to an Artifact Registry (and Container Registry).
• It would be nice to have some documentation for new contributors of this repo.
• The documentation should cover:
  • why we use them Cloud Build
  • a walkthrough of one of the cloudbuild.yaml files
  • how Cloud Build works (briefly)
  • how to access the Cloud Build Triggers
  • how to navigate Cloud Build Triggers "Build" in this repo
• Thanks, @ahrarmonsur, for inspiring this ticket!

[NimJay]

I've just created a Google-internal document about this repository's CI/CD.