move version picker higher on the sidebar

[akeimach]

why

what changed

test plan

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Summary by cubic

Moves the docs version picker to the top of the sidebar and restores the v2/v3/v4 dropdown for quick version switching. Addresses Linear STG-2802 by making the version selector visible again after the v4 docs changes.

• New Features
  • Adds a custom version dropdown (v4, v3, v2) below the search bar in the sidebar via packages/docs/version-switcher.js.
  • Hides the native bottom-of-sidebar Mintlify selector on desktop.
  • Matches the v3 language dropdown styling and behavior (ARIA, Escape to close, outside-click to dismiss).
  • Updates automatically on client-side navigation; integrates with existing language selector visibility rules.

^{Written for commit 74067f2. Summary will update on new commits.}

[changeset-bot]

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Latest commit: 74067f2

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