add-prerequisites

Created by: JeanPaulSG

Add reference to 4. configuring the yaml file and 5. publishing changes to the customer's branch, before customising and executing the startup script.

Made prerequisite and preparing a fork section common across all three cloud specific docs.

Issue: QA Cloud Docker Compose

Test plan

Technical review for accuracy by Kevin or Crystal. Peer review by @blvckcoffee or @dulanism proof read please.