[Docs] Add CLI reference doc for `vllm bench sweep plot_pareto` (#29689)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/cli/bench/sweep/plot_pareto.md

+# vllm bench sweep plot_pareto
+
+## JSON CLI Arguments
+
+--8<-- "docs/cli/json_tip.inc.md"
+
+## Arguments
+
+--8<-- "docs/argparse/bench_sweep_plot_pareto.inc.md"