Resolve "Update auto devops flow to support multi-cluster setup"

What does this MR do?

Places new documentation in both Auto DevOps settings as well as Kubernetes "add cluster" screen on how users can configure multiple clusters with Auto DevOps and Kubernetes.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/5956