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

   GenAI-quickstart/genai/api/npc_chat_api/config.yaml

   Lines 24 to 27 in 2f954e2

   	# GenAI provider - GKEGenAI or VertexAI. Note that switching GenAI implementations switches the
   	# embedding model requiring a data regeneration using the /reset_world_data endpoint.
   	genai = "GKEGenAI"
   	# genai = "VertexAI"