docs: make the benchmark checklist item conditional on touching benchmarked code (#8141)

Suggested by @HarelM after #8122 merged: https://github.com/maplibre/maplibre-gl-js/pull/8122#issuecomment-5233617539

"Post benchmark scores" asks every PR for something most PRs have no way to produce, so it gets skipped without a thought. Stating the condition makes it answerable: if you touched a file with a `*.bench.ts` next to it, run `npm run bench` before and after and post the results.

The pointer to `test/bench/README.md` is where the baseline and compare commands live, so the checkbox and the README now say the same thing.

- [x] No backports from Mapbox projects
- [x] Related: #8122
- [x] No benchmark scores: template text only
- [x] No CHANGELOG entry, per the CONTRIBUTING rule for documentation changes
- [ ] Confirm you have read our AI policy [here](https://github.com/maplibre/maplibre/blob/main/AI_POLICY.md)