[docs] Add architecture diagrams

[ananthsub]

Fixes #292

This PR covers the rollout collection within NeMo Gym for standalone usage (ie not used in conjuction with an RL training framework). For the NeMo RL + Gym integration summary, I will add docs to the NeMo RL page, and update Gym docs with a pointer to those for reference.

These docs cover:

• Control plane: ng_run startup sequence (CLI parsing, config loading, Ray init, server spawning)
• Server architecture: Head server, uvicorn/FastAPI initialization
• HTTP request flow: Example rollout showing Agent -> Model -> Resources interactions
• Data plane: ng_collect_rollouts flow starting from the headserver discovery

This change also adds an extra dependency on sphinxcontrib.mermaid for mermaid diagrams to render in the docs page

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