Improve wording around what clusters are and how to use them in the service provider

[Skarlso]

Once a service provider is starting to be built it's not really clear which cluster the user is supposed to use and what client and from where.

There are nice graphics here and there about the onboarding cluster and the provider cluster and the mcp cluster and the workload cluster... but you are quite lost to understand in the code on which to use at what point.

Also, right now, since workload clusters aren't really recommended yet, we get into this weird limbo of where I'm supposed to do things. :)

Now, I worked on CAPI/CAPA and various cluster generator tools, so it was a bit easier for me to understand the difference between a control plane and a workload cluster. But the distinction between the platform / onboarding and mcp needs to be a bit clearer with a simple example talking about what the cluster is from the developer's perspective and what it is from the user's perspective.

So me, a service provider developer, can understand more precisely which clients to use and where my objects will end up at.