Update AIGW examples to use the new kongctl nested lookup

[jbaross]

Jobs to be done (optional)

Currently AIGW examples either use either {% entity_examples %} blocks and hardcode the tutorial gateway name or use plain md code blocks and rely on bash to merge in an environment variable for a dynamic gateway name.

Following https://github.com/Kong/kongctl/blob/main/docs/declarative.md#nested-yaml-tags we can now have dynamic gateway names in entity examples using:

ai_gateway: !lookup {id: !env AI_GATEWAY_ID}

Definition of done

All codeblocks use the new format

Size

• M (Medium) Example: writing a UI how to for a straightforward product, adding a plugin example