docs: refresh the performance claims

[tannevaled]

• index.md: the Slower bullet still said MatMul 0.55× at 1024² / 0.3× at 256² and Dot/mat·vec 0.16×, all from before v0.6.0. It now gives the current same-run figures, and adds the RISC-V X60 and IBM z17 results.
• performance.md header: adds one paragraph for riscv64 and s390x. The page body is the mirror of perf.md and is unchanged.

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