Update docs for v0.1.8 API

[anakin87]

Description

I noticed that the docs are not up to date with the v0.1.8 API (see release notes).

In this PR, I'm updating the docs to reflect the changes (use @vf.stop, follow updated env_response signature, ...)
This also fixes #608.

@willccbb @mikasenghaas

Note that I did not update api_reference.md because it's unclear to me what should be exposed there. I'd prefer if a maintainer could handle this separately.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

---

Note

Updates docs to match v0.1.8 APIs and patterns.

• Switch env_response to return Messages (no (messages, state) tuple) across examples; remove explicit turn handling in favor of state["trajectory"]
• Introduce @vf.stop decorators for stop conditions (e.g., game_over, task_complete); remove prior is_completed usage and guidance
• Expand state schema and rollout tracking: add trajectory, completion, is_completed, stop_condition, detailed timing; document TrajectoryStep structure
• Revise scoring/output fields: rewards now stored in state["reward"], metrics in state["metrics"]; update evaluation lifecycle and training data examples accordingly
• Cleanup examples (Wordle, MultiTurnEnv, ToolEnv) to new signatures and messaging; minor text/link fixes (e.g., training links, save_every description)

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[lukalotl]

Finally found this PR after digging around— the docs really tripped me up over the last few hours, would be great to get this in

[mikasenghaas]

lgtm

[anakin87]

@mikasenghaas Feel free to merge...

As a contributor, I cannot do that.