docs: performance and security mirrors at v0.4.1

[tannevaled]

Re-syncs performance.md from docs/perf.md and security.md from SECURITY.md of go-ndarray/ndarray v0.4.1.

• Parallel GEMM. New section on the diagnosis and the new driver (perf(gemm): start workers once per product, synchronise on work done ndarray#26). The earlier claim based on the cycles profile is corrected. The summary now gives the OpenBLAS-16 ratios measured in one run.
• Security. Adds the differential fuzzer and the memory-amplification caveat (NumPy parity: Sum/Prod over an empty axis, Slice with fewer indices; differential fuzzer ndarray#25).

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