[docs] Tabs examples with motion

[mj12albert]

https://github.com/mui/base-ui/issues/1494 https://github.com/mui/base-ui/issues/1486

Demo: https://deploy-preview-1613--base-ui.netlify.app/experiments/tabs/motion

• Used mode="wait" on AnimatePresence, the entering child waits until the exiting child has animated out, other modes should be possible, though extra display/positioning styles would be required (set via motion) to prevent overlap:
• If using Tailwind v4, setting hidden={undefined} on Tabs.Panel is required to ensure the panel is visible during the exit animation (same workaround as Collapsible)
• Replicating the indicator transition from the docs styles with motion turns out to be tricker than the panel, it can't seem to transition the same CSS variable that changes value:

// doesn't work even with the entries repeated in `initial`/`exit`
<Tabs.Indicator render={
  <motion.div
    animate={{
      translate: 'var(--active-tab-left) -50%',
      width: 'var(--active-tab-width)',
    }}
  />
}

• [x] I have followed (at least) the PR section of the contributing guide.

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[mj12albert]

Closing this since motion has examples now