Add inference pool content routing guide

[danehans]

Summary

• Update the inference quickstart to use llm-d-inference-sim.
• Add a multiple InferencePools guide that uses AgentgatewayPolicy content routing without an external BBR payload processor.
• Link the new guide from related inference and content-routing docs.

Testing

• git diff --check
• Validated on a fresh kind cluster with GAIE/EPP v1.5.0, agentgateway, Qwen3 and DeepSeek simulator InferencePools.
• Verified /v1/chat/completions returned 200 for Qwen/Qwen3-32B, deepseek/DeepSeek-r1, and movie-critique.