Migrate Benchmarking guide to use InferencePerf

[kfswain]

The guide here: https://gateway-api-inference-extension.sigs.k8s.io/performance/benchmark/ uses the old LPG tool, the benchmark tool we canonically use is https://github.com/kubernetes-sigs/inference-perf

CC: @jjk-g @achandrasekar @ahg-g