docs: add NVIDIA NeMo Gym integration guide

[max-sudolabs]

Summary

Adds an integration guide for NVIDIA NeMo Gym under Agents, documenting how to run NeMo Gym agent-evaluation and training environments in E2B sandboxes via its new e2b sandbox provider.

The page covers:

• installing the nemo-gym[sandbox] extra and setting E2B_API_KEY;
• the shipped provider config (nemo_gym/sandbox/providers/e2b/configs/e2b.yaml) and how agents reference it via sandbox_provider: sandbox;
• running an agent harness (mini_swe_agent_2) against E2B with gym env start;
• building E2B templates from OCI images with NeMo Gym's provisioning CLI;
• security notes (secure envd, internet-access toggle, API-key stripping from serialized worker configs, nemo-gym/<version> User-Agent attribution).

Also registers the page in the Agents nav group in docs.json.

Context

• Upstream NeMo Gym E2B provider PR: feat(sandbox): add E2B sandbox provider NVIDIA-NeMo/Gym#2301 (adds the e2b provider this guide documents).
• Linear: INT-93 — Explore AI integration angle for NVIDIA coding agent workflow.

Draft — blocked on upstream merge

Opening as draft because the guide links to NVIDIA-NeMo/Gym@main, which only resolves once Gym#2301 merges. Ready to mark ready-for-review once that lands.

Verification

• All commands, config paths, CLI flags, and the deterministic template name (task-1-0__f00ee9d593d9) were validated against the NeMo Gym source on the provider branch.
• A placeholder FontAwesome dumbbell icon is used; happy to swap in a NeMo Gym logo SVG if preferred.