ci(bench): publish benchmark time series to /bench on gh-pages

[jack-champagne]

Summary

• Publishes a live time-series benchmark dashboard at https://harmoniqs.github.io/CuQuantum.jl/bench/.
• Coexists with Documenter on the same gh-pages branch — Documenter only writes /dev, /stable, /vX.Y.Z, /previews, so /bench is out of its way.
• PR and branch runs stay out of the time series (per request).

How it works

• `benchmark/run_benchmarks.jl` now emits `benchmark/results/bench.json` alongside the existing CSV, in `customSmallerIsBetter` shape (`{name, unit, value}`).
• `gpu-benchmark.yml` runs `benchmark-action/github-action-benchmark@v1` after the GPU job.
  • `save-data-file` and `auto-push` are `${{ github.event_name == 'push' && github.ref == 'refs/heads/main' }}` — only push-to-main appends to the series.
  • PR / workflow_dispatch runs still fetch the baseline, render a comparison, and comment on regressions via `comment-on-alert`, but don't persist.
• `docs/src/benchmarks.md` gets a dashboard link at the top.

Follow-ups to consider (separate PRs)

• Add `push: branches: [main]` or a `schedule` trigger — today the series only grows when someone `workflow_dispatch`es the benchmark manually against main.
• Prune `data.js` once it gets long (years out).

Test plan

• Merge to main, then `workflow_dispatch` a benchmark run against main to seed `/bench/data.js` on `gh-pages`.
• Visit https://harmoniqs.github.io/CuQuantum.jl/bench/ and confirm the chart renders.
• Trigger a run against a PR branch; confirm no commit lands on `gh-pages`, but if the benchmark crosses 120% of baseline we get a PR comment.
• Confirm Documenter `docs/deploy` still works and the dashboard link resolves from the rendered docs.