docs: Add NeMo Gym to the Environments guide

## Summary

The "Environments for GRPO Training" guide (`docs/guides/environments.md`, published at https://docs.nvidia.com/nemo/rl/latest/guides/environments.html) covers the Math, Code, Code Jaccard, and Reward Model environments plus custom-environment registration. It does not yet include the NeMo Gym integration. Adding it would let readers discover NeMo Gym from the page where they go to choose how to set up an environment for training.

## Why this is high-value

NeMo Gym is the recommended path for using environments with NeMo RL, and the battle-tested path used for production Nemotron training. Because of that, it should be featured prominently in the environments guide — this is the page where users decide how to set up an environment.

NeMo Gym is currently documented only in `docs/design-docs/nemo-gym-integration.md` (an architecture/design doc). Linking and summarizing it from the environments guide would put the recommended path where users are already looking.

## Proposal

Add a **"NeMo Gym Environments"** section to `docs/guides/environments.md` that:

- Briefly explains what NeMo Gym is and when to use it
- Shows the minimal enabling config (`env.should_use_nemo_gym: true`, `async_engine` + `expose_http_server`, `nemo_gym.config_paths`) — condensed from the design doc.
- Points to `examples/nemo_gym/` and links both the design doc (`docs/design-docs/nemo-gym-integration.md`) and the NeMo Gym docs site.

## References

- `docs/guides/environments.md` — target page for the new section
- `docs/design-docs/nemo-gym-integration.md` — existing dev-facing doc
- NeMo Gym → `reward1..K` multi-reward bridge issue https://github.com/NVIDIA-NeMo/RL/issues/2892