AIGW Policy scopes: add FAQ and update corresponding sections

Definition of done

Add an FAQ and a few sentences explaining how it works to https://developer.konghq.com/ai-gateway/entities/ai-policy/#ai-policy-scopes and https://developer.konghq.com/ai-gateway/entities/ai-model/#ai-policy-execution-order

Information

Problem: hen a policy is scoped to model, it will not get triggered

Reported in slack

The reason for this is the order of operations for global vs model scoped policies.
When using the global scope (left screenshot) the pre-function is executed before authentication
When using the model scope (right screenshot) the pre-function is executed after authentication

and

The request must first be authenticated before routing to a specific model. As a result, to execute a model scoped plugin/policy the request first must authenticate and then it can execute the plugin

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