docs: Improve NeMo Gym discoverability across NeMo RL entry points

[anwithk]

Problem

NeMo RL supports NeMo Gym, but users entering through NeMo RL’s primary documentation pages are unlikely to discover the integration or understand when to use it.

An integration guide already exists, so this issue focuses on connecting and clarifying existing documentation rather than creating new workflows.

Scope

• README: Add NeMo Gym to the Overview with a short description and link to the integration guide.
• Overview: Explain how NeMo RL and NeMo Gym complement each other.
• Evaluation: Mention NeMo Gym evaluation capabilities and clarify when to use NeMo RL-native evaluation versus NeMo Gym.
• Environments: Tracked in docs: Add NeMo Gym to the Environments guide #2893
• Documentation index: Make the NeMo Gym integration guide easier to discover.
• Standardize the product name as NeMo Gym throughout the touched pages.

Acceptance criteria

• Users can discover NeMo Gym from the README, Overview, Evaluation, and Environments pages.
• Each page briefly explains why a NeMo RL user might need NeMo Gym.
• Relevant pages link to the existing NeMo Gym integration guide and NeMo Gym documentation.
• Existing NeMo RL-native evaluation and environment documentation remains available.

Follow-up work

Track separately:

• Validate shared NeMo RL and NeMo Gym training JTBDs.
• Create end-to-end tutorials spanning both products.
• Improve the cross-product DX review and triage process.

[judyheflin]

I'd also add improve discoverability for existing RL x Gym recipes: https://github.com/NVIDIA-NeMo/RL/tree/main/docs/guides/models/nemotron