docs: add v8 to v10 upgrade guide

[gpbl]

What Changed

• Expanded the stable upgrade guide into a clearer v8-to-v9 migration path.
• Kept the next v9-to-v10 guide focused on v10-only removals and package/import changes.
• Added a separate next-version guide for direct v8-to-v10 upgrades.
• Grouped the next-version upgrade pages under an Upgrading guide sidebar category.

Review Notes

The direct v8-to-v10 guide was checked against the v8 docs surface so it avoids v9-only calendar import guidance and includes v8-era APIs such as useInput, useNavigation, useDayRender, and the old provider/context exports.

Validation:

• pnpm exec prettier --check apps/website/docs/upgrading.mdx apps/website/versioned_docs/version-next/upgrading.mdx apps/website/versioned_docs/version-next/upgrading-v8-to-v10.mdx apps/website/versioned_sidebars/version-next-sidebars.json
• git diff --check -- apps/website/docs/upgrading.mdx apps/website/versioned_docs/version-next/upgrading.mdx apps/website/versioned_docs/version-next/upgrading-v8-to-v10.mdx apps/website/versioned_sidebars/version-next-sidebars.json
• pnpm --filter website run build

The Docusaurus build succeeds with the existing non-blocking update-check warning about /Users/gp/.config permissions.

[changeset-bot]

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Latest commit: 960699f

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