docs: add MIGRATING.md from the llm.winlab.tw move

[zyx1121]

What moving llm.winlab.tw (NYCU-WinLab/llm.winlab.tw#80) taught, written down for the next app (WinLab portal is next):

1. Install steps for an existing shadcn app
2. What replaces card, sidebar, sheet, the theme toggle, chart-N, the old badge and button variants, type sizes and radii
3. Radix to Base UI behavior that silently breaks: asChild, type="submit", link buttons getting role="button", dialog dismissal, onSelect, Select items, nullable onValueChange
4. Tests: paused clocks freeze Base UI's open and close (with closeDialogs / openSelect helpers), headless Chromium crashing on the CJK font slices, the narrow-screen page menu

README links to it from Use it.

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