Docs: treat KTO as a stable trainer

[qgallouedec]

KTOTrainer was promoted to the stable API (#6175), but the docs and docstrings still treated it as experimental. This aligns KTO's visibility and references with DPOTrainer.

Changes

• What's New (README.md, docs/source/index.md): add a "KTO is now stable" highlight.
• Taxonomy (docs/source/index.md): drop the stale 🧪 marker from KTOTrainer and move it next to DPOTrainer.
• README parity with DPO: add KTOTrainer to the Highlights trainer list, a KTOTrainer code example, and a trl kto CLI example.
• Stable references: replace all experimental.kto.KTOTrainer / experimental.kto.KTOConfig refs with KTOTrainer / KTOConfig across docs (kto_trainer.md, example_overview.md, paper_index.md, dataset_formats.md) and source docstrings (kto_config.py, kto_trainer.py), including the collator paths (~trainer.kto_trainer.DataCollator...).

No code behavior changes; the trl.experimental.kto deprecation shim is untouched.

---

Note

Low Risk
Documentation and docstring-only updates; training behavior and the trl.experimental.kto shim are unchanged.

Overview
Documentation and docstrings now present KTO as a stable trainer alongside DPO, matching the API promotion in #6175 (no runtime changes).

What's New in README.md and docs/source/index.md adds a KTO is now stable highlight; the docs taxonomy moves KTOTrainer out of experimental (drops 🧪) and lists it with offline methods next to DPOTrainer. The README gains KTO parity with DPO: Highlights trainer list, a KTOTrainer quick-start, and a trl kto CLI example.

Across docs (kto_trainer.md, dataset_formats.md, example_overview.md, paper_index.md) and source (kto_config.py, kto_trainer.py), references switch from experimental.kto.KTOTrainer / KTOConfig to stable KTOTrainer / KTOConfig, including autodoc targets and collator paths under ~trainer.kto_trainer.*. dataset_formats.md lists KTOTrainer in the main trainer table instead of under experimental.

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