Provide write-up for APIM scenarios with AI hub

## Goal state

We need a detailed article that describes the various scenarios where APIM is adding value in context of Azure OpenAI.
Besides dynamic load balancing and retry/throttling logic etc., we must provide prescriptive guidance for known use-cases, such as:
* Combination of PTU and pay-as-you-go.
* Same as the above, where organisations can "start small & expand" without any dramatic changes required on the application side.
* Differen APIM policies to address different scenarios. We should ensure this is reflected and available as options in the reference implementation as we wrap things up.
* External vs Internal vs Hybrid
* Single region - and multiple regions