[codex] Add sandbox API docs guide

[hemildesai]

Summary

• Add a Fern guide for the provider-neutral nemo_gym.sandbox API.
• Link the guide from the Infrastructure docs and include nemo_gym.sandbox in the API reference index.
• Adjust Engineering Notes ordering so the new Sandbox API page appears in the Infrastructure section.

Validation

• npm run check -- --warnings passed with 0 errors.
• Remaining Fern warning: redirects check skipped because the local session is not authenticated with Fern.

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[github-actions]

🌿 Preview your docs: https://nvidia-preview-codex-opensandbox-coverage-tests.docs.buildwithfern.com/nemo/gym

Here are the markdown pages you've updated:

• nemo/gym/api/reference/api-reference
• nemo/gym/infrastructure/engineering-notes
• nemo/gym/infrastructure
• nemo/gym/infrastructure/sandbox/adding-a-provider
• nemo/gym/infrastructure/sandbox

[cwing-nvidia]

Given we will be adding a number of providers, can we update to a parent-child content structure:

infrastructure/sandbox/index.mdx — provider-neutral overview: when to use it, core types, lifecycle, sync vs async, SandboxSpec fields, file transfer, status/cleanup, error handling, a local first-run example. This is the part every user reads.

infrastructure/sandbox/opensandbox.mdx — OpenSandbox-specific: connection/create/probe/operations config, provider_options, resource→k8s mapping, isolation/security properties, the mini_swe_agent_2 YAML + ng_run command, and OpenSandbox troubleshooting rows.

(ECS Fargate, Apptainer, OpenShell etc providers will each have their own child page)

infrastructure/sandbox/adding-a-provider.mdx — the SandboxProvider protocol + registry.py contributor guide.

Each new provider from #1685 then drops in as one more child page with the same shape (setup / config / isolation properties / example)

[cwing-nvidia]

i thought @ananthsub 's PR had decoupled this?

[hemildesai]

Yes removed for now. Will add a configuration guide after Ananth's PR is merged