📜 Update README and doc index

[qgallouedec]

What does this PR do?

• Add Open-R1 blog post in the index page of the doc
• Reorganise README/Doc to highlight our top trainers (SFT, GRPO, DPO, Reward)
• Drop some old content

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

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