[rllib] Document creating an ensemble of envs; also add vector_index attribute to env config

[ericl]

What do these changes do?

This also removes the async resetting code in VectorEnv. While that improves benchmark performance slightly, it substantially complicates env configuration and probably isn't worth it for most envs.

This makes it easy to efficiently support setups like Joint PPO: https://s3-us-west-2.amazonaws.com/openai-assets/research-covers/retro-contest/gotta_learn_fast_report.pdf

For example, for 188 envs, you could do something like num_envs: 10, num_envs_per_worker: 19.

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