Docs: Set init/upgrade snippets to next on next

[shilman]

Setting the init/upgrade snippets to next so that we can have users test prereleases like @storybook/react-native-web-vite. @kylegach / @jonniebigodes we need to add a release checklist item to ensure that these are always latest on main.

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Greptile Summary

Updates initialization and upgrade command snippets in documentation to use @next tag instead of @latest, enabling users to test prerelease versions of Storybook packages.

• Modified docs/_snippets/init-command.md to use @next tag for npx, pnpm, and yarn initialization commands
• Modified docs/_snippets/storybook-upgrade.md to use @next tag for upgrade commands across all package managers
• Requires new release checklist item to ensure commands use @latest on main branch

[greptile-apps]

LGTM

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