Update docs: add What's New in v0.8 with actual release content

[YunchaoYang]

Replace the placeholder v0.8~ doc (which was a copy of v0.5) with the real v0.8 release notes: new models (Gemma3n, OLMo2/3, generic HF), S3 checkpoints, context managers, PyTorch 2.9/2.10, breaking changes, and the maintenance mode announcement.

Update index.rst to show v0.8 as the latest news (keeping v0.5 as historical reference).

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Fixes #{issue number}

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• Was the content of this PR discussed and approved via a GitHub issue? (no need for typos or documentation improvements)
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