docs: add blog post for multi-stage delivery

[bacherfl]

This PR adds a blog post describing how Keptn can be used to implement multi stage promotion.

Closes #3019

[StackScribe]

Initial comments after a quick read-through. The specific details of what one needs to do are nicely done but I kind of get lost reading through it. For starters, I'd like the Abstract to give a high-level overview of how all these technologies work together and what each contributes. Also, you start out talking about following a workload then end up talking about applications, and we know that "application" has about a zillion meanings in this world. Is an ArgoCD application the same thing as a Keptn application or does a Keptn application include multiple ArgoCD applications? Remember that the Kubernetes docs define a workload as an application.

As you get into the implementation specifics, I kind of get lost. It might help to have a high-level list of what we are going to do at the beginning, just to get me oriented. So something like:

• Set up the environment
  • Set up a GitHub repository with an access token, GitHub, workflows, and GitHub actions
  • Prepare a Kubernetes namespace for each stage (dev and production)
  • Prepare the ArgoCD applications with an appropriate Helm chart for each
  • Apply labels to associate the Deployment resource with the KeptnWorkLoad resource
  • Define Keptn pre-/post-deployment checks and tasks
  • Define the metadata to be passed through the trace
  • Define a TraceParent that links the deployment traces of the production stage to those of the dev stage
• Run the production flow...

That's quite rough and probably wrong and not as "rich" as it would be in your voice,,, But a start.

[sonarqubecloud]

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