rollout notebook: fixups to benchmarks, plots, text

1. MJX benchmarks use jax.lax.scan and block_until_ready
2. Benchmark parameters updated to match RL use case
3. humanoid benchmarks are warmstarted by 2 seconds, humanoid100 by 4 seconds
4. benchmarks use skip_checks and preallocated threadpools
5. Improved advanced usage plots
6. Benchmark no longer includes model initialization in timed section
7. Update notebook text as needed

Author: aftersomemath