docs: add documented BF16 sparse benchmark runner script
tools/run_bf16_sparse_bench.sh documents and runs the exact BF16 sparse SDPA
benchmark command: sources oneAPI 2026.1, pins a GPU via ZE_AFFINITY_MASK,
uses the rebuilt ark-xbuild extension, and saves a timestamped CSV + log under
benchmarks/results/. Includes env overrides and prerequisites in the header.
