Add reproducibility docs for review-deploy job locally

Context

General

Changing the review-apps CI/CD pipeline is scary:

• It's used throughout the company.
• We don't have an easy way to test it locally
• It's slow to rely on the CI/CD pipeline to make a change

Personal

I've been working on gitlab-org/gitlab!85520 (merged) and gitlab-org&7826 (closed), which required some changes to the review-apps CI/CD code. I then realized that I needed a way to iterate rapidly locally.

What does this MR bring?

Documentation on how to simulate the review-deploy job locally. Specifically, it allows you to:

1. See the helm upgrade command that would be used to deploy the review app
2. See the helm values that would be passed in the helm release when deploying to Kubernetes
3. Compare the behaviour of your branch against the master branch