docs(annotate_subtasks): document Gemini Robotics-ER 1.6 support

[WilliamYue37]

What this does

Follow-up to #255 — that PR added Google's Gemini family (including Gemini Robotics-ER 1.6) as a --model option for src/opentau/scripts/annotate_subtasks.py but did not update the docs. This PR brings docs/source/tutorials/datasets.rst in sync with the implemented behavior:

• Top-of-section blurb now mentions that Gemini is supported alongside Claude.
• "Prerequisites" lists GEMINI_API_KEY / GOOGLE_API_KEY next to ANTHROPIC_API_KEY.
• "Running the script" gains a Gemini Robotics-ER 1.6 invocation example.
• The --model row in the flags table explains the Anthropic-vs-Gemini routing rule (IDs starting with gemini or robotics-er are dispatched to google-genai).
• API cost estimate is annotated as Claude-specific (Gemini pricing differs).

📝 Documentation

How it was tested

• uv run pre-commit run --files docs/source/tutorials/datasets.rst — passes.
• Manually re-read the flag descriptions against src/opentau/scripts/annotate_subtasks.py to confirm the routing rule (_is_gemini_model) and env-var fallback (GEMINI_API_KEY → GOOGLE_API_KEY) match.

How to checkout & try? (for the reviewer)

```bash
git checkout docs/annotate-subtasks-gemini

Render the docs locally

uv run sphinx-build -b html docs/source docs/_build/html
open docs/_build/html/tutorials/datasets.html
```

Checklist

• I have added Google-style docstrings to important functions and ensured function parameters are typed.
• My PR includes policy-related changes.
  • If the above is checked: I have run the GPU pytests (pytest -m "gpu") and regression tests.

🤖 Generated with Claude Code

[claude]

[claude-review] summary for commit 0e61085

No blocking issues found.