Lab package documentation

Benchmark engine, pipeline steps, and experiment design docs live next to the mas-lab package.

Topic	Path in repository
Lab package index	lab/docs/index.md
Benchmarking	lab/docs/benchmark.md
Designing experiments	lab/docs/designing-experiments.md
Pipeline steps	lab/docs/pipeline-steps.md
Labs quickstart	lab/docs/labs-quickstart.md

YAML field reference for experiments, datasets, and pipelines: Specifications.