README: quick start first, one command per mode

Quick start moves to the top of the README, straight under the demo gif:
clone, then one command per mode, with the batch/single comparison table
right below the choice so the decision and the command sit together. The
plain `docker run` one-liner is documented as the no-compose, no-checkout
path, with a full section in docs/docker.md (env-flag mapping, named-volume
model storage, mode switching); docs/docker.md also drops the now-redundant
`docker compose pull` step. The bench-harness quoting note under the results
table is removed.