docs(readme): swap the benchmark hero image for the dashboard scoreboard view

Replace docs/website-src/images/benchmark-dashboard.png (the dashboard header
card) with the Suite Scoreboard + Dtype Heatmap view, which shows the concrete
per-suite geomeans (Statistics 2.24x, Reduction 1.81x, ... Manipulation 0.40x)
and the per-dtype cache-tier wins. The header's snapshot chips and the four
headline numbers are already reproduced in the Performance text table, so the
scoreboard is the higher-signal single visual. The README <img> reference and
alt text are unchanged.

Author: Nucs