Docs: compare environment integrations (OpenEnv, OpenReward, Harbor)

[sergiopaniego]

What does this PR do?

Adds a short comparison of TRL's three environment integrations and wires the trainer docs to it, so a user navigating from the GRPO docs can find the environments overview and pick the right integration.

Depends on huggingface#6018

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? N/A (docs only).

AI writing disclosure

• No AI usage: the PR was written entirely by a human.
• AI-assisted: some parts were suggested or improved by AI, but the PR was written and reviewed by a human.
• AI-generated: the PR was mostly or fully generated by an AI tool.

Who can review?

@qgallouedec @adithya-s-k