Update the self-managed and SaaS feature parity princple

Why is this change being made?

We have recently started plans for features which at least in the initial iterations require SaaS services to work. These include Analytics, Observability, as well as some of our AI functionality. We should update our SaaS and self-managed principle to document when and why can happens. I have tried to do that here, and give some guidance that these patterns should generally be avoided unless really needed.

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