Add reference documentation outlining the cluster architecture

[NucciTheBoss]

Now that we have the Underlying projects and dependencies reference documentation which paints a better picture of all the different bits and bytes that come together to form a Charmed HPC cluster, we also need reference documentation that helps users create an accurate mental model of what a Charmed HPC cluster looks like. We should have a diagram that outlines how the Kubernetes and Machine clouds play together, and where the different Juju applications are located within the cluster.

Mermaid currently isn't an option because the accessibility isn't great, so we'll have to come up with something for rendering what the Charmed HPC cluster architecture is. I have this diagram from previous presentations that outlines where we're going:

However, it would be great to have something that maps out all the integrations between the different deployed applications and how they talk to each other.