Clarify how resources work on Upsun

[nicogommen]

Where on docs.platform.sh should be changed?

On Upsun documentation website (https://docs.upsun.com/), we'd like to better describe how resources work on our Grid infrastructure.
A bit similarly to what we are doing in the new Dedicated pages (https://docs.platform.sh/dedicated-environments/overview.html#grid-vs-dedicated).
For example, mention wherever relevant that the projects on Upsun are running on containers with dedicated resources on top of a shared redundant infrastructure.

What exactly should be updated?

Not sure yet which pages would be the most relevant.
We first need to analyze where exactly this information would fit best.

Additional context

We might offer more types of resources on Upsun at some point (dedicated resources, guaranteed resources, performance CPU, ...etc.), so we will need to introduce and explain the differences between the new type and the current standard resources. Adding this clarification about the current resources now already would be helpful in anticipation for when will have more types available.