docs: explain how argo cd app health factors into stage health

[krancour]

This follows up on #2921, which added a new, dedicated page about Argo CD integration.

One key aspect of the integration between the two systems that causes a lot of confusion is health checks.

Example: #3155

In short, Kargo isn't exclusively Argo CD centric. Just because a Stage's underlying App(s) is/are healthy does not automatically mean the Stage is healthy. Other things can be wrong._ (Although the inverse is true. An unhealthy App automatically implies an unhealthy Stage.)

Let's please enhance the Argo CD integration with a section about health checks to call this detail out. There is also a section on health checks in the argocd-update section of the promotion steps reference and it should link to the new section of the Argo CD integration page so users of that step can easily find more details.