Additional Documentation on Which Endpoint use Vertex vs GKE

[igooch]

We got feedback from user that it is not clear which endpoints call Vertex vs. which endpoints call a LLM on the GKE cluster, and how to switch between the two.

We should:

1. Update documentation to make clear which endpoints call Vertex (/genai, /genai/chat, /genai/code, /genai/image, /genai/text, etc.)
2. Update documentation for instructions on how to switch between running a LLM on GKE (current default) and running on Vertex in the NPC chat I https://github.com/googleforgames/GenAI-quickstart/blob/2f954e27be98b1a691610d1e1da4a10bd9a741de/genai/api/npc_chat_api/config.yaml#L24-L27