Add benchmark how-to documentation

[amd-dlimpus]

Summary

• Add a validated benchmark how-to for the fp8 scale-kernel branch.
• Put the vLLM benchmarks/multi_turn path first, followed by the internal kv-cache-bench LongCodeBench flow.
• Document runtime/client environment split, GPU memory/GMU guidance for MI300X and MI355X, cleanup, validation, and common failure checks.

Test plan

• Validated Benchmark 1 end-to-end: 180 requests, 180 completed with output, 0 failures.
• Validated Benchmark 2 end-to-end using BENCH_SECONDS=600: summary had nonzero throughput/output tokens and zero errored turns.
• Ran markdown/lint diagnostics in Cursor with no issues reported.

Made with Cursor

[github-actions]

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