[Doc] Reorganize benchmark docs (#29658)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

docs/benchmarking/README.md

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+# Benchmark Suites
+
+vLLM provides comprehensive benchmarking tools for performance testing and evaluation:
+
+- **[Benchmark CLI](./cli.md)**: `vllm bench` CLI tools and specialized benchmark scripts for interactive performance testing.
+- **[Parameter Sweeps](./sweeps.md)**: Automate `vllm bench` runs for multiple configurations, useful for [optimization and tuning](../configuration/optimization.md).
+- **[Performance Dashboard](./dashboard.md)**: Automated CI that publishes benchmarks on each commit.