tutorial: secure access to review apps

Is your feature request related to a problem? Please describe.

Review Apps were pioneered as a concept by GitLab and I personally used them in a mid-size team environment and they were absolutely beloved by the team as it was possible to review and comment on each PR. One drawback was that review app was talking to staging DB with actual data in it, and although it was hard to guess the URL (that was app-name-commit-hash.some-internal-domain.com) it still wasn't fully comforting.

Having Pomerium in front of review app would allow team to secure and restrict access to the review app.

Describe the solution you'd like

We should make a guide (could be non-GitLab centric though) how to use Pomerium with auto-deployed application versions. In fact, there is just one kind of such environment where we support dynamic route creation, and it is Kubernetes, which also works fine for review app deployment and subsequent cleanup once PR is merged.

We should try to get into the following open source guides:

1. GitLab - https://docs.gitlab.com/ee/ci/review_apps/#review-apps-examples
2. Jenkins https://github.com/jenkins-x/jx-docs/blob/master/content/developing/preview.md
3. ArgoCD seems to have a way but it's not on their website? found this https://codefresh.io/continuous-deployment/creating-temporary-preview-environments-based-pull-requests-argo-cd-codefresh/
4. CircleCI has standard Kubernetes ORB that makes it easy to create Ingress resources straight from CircleCI https://circleci.com/developer/orbs/orb/circleci/kubernetes

probably there are many more...

Start with the linked GitLab Review Apps examples and the Kubernetes dynamic route-creation approach described in the issue. Compare the Jenkins, ArgoCD, and CircleCI references to determine the guide's scope. Done means the documentation explains how to secure auto-deployed review applications with Pomerium and covers the supported Kubernetes workflow and cleanup after a pull request is merged.