docs: explain partial rollout checkpoint states

[ananthsub]

Summary

• distinguish logical rollout, agent execution, checkpoint phase, and model admission state
• explain failure handling and normal-path versus checkpoint-time overhead
• compare restart, drain, replay, central storage, uncoordinated snapshots, and process snapshots with the participant design
• align the API inventory with the Gym stack ending at feat(checkpoint): adapt initial stateful environments NVIDIA-NeMo/Gym#2946 and call out remaining production gaps

Test plan

• parse the HTML with Python's standard HTML parser
• parse all embedded JavaScript with Node.js
• verify the document diff has no whitespace errors

The Pages workflow is intentionally not triggered by this PR. Publication remains a manual workflow dispatch.