docs(legacy-stack): add deprecation warning (#2095)

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## Summary
<!-- Please brief explanation of the changes made -->

Add deprecation warning to LegacyStack storybook docs

## Details
<!-- Please elaborate description of the changes -->

스토리북 문서에 사용 금지 경고 문구를 추가합니다.

### Breaking change? (Yes/No)
<!-- If Yes, please describe the impact and migration path for users -->

No

## References
<!-- Please list any other resources or points the reviewer should be
aware of -->

-
https://storybook.js.org/docs/writing-docs/mdx#linking-to-other-stories-and-pages

packages/bezier-react/src/components/LegacyStack/LegacyStack.mdx

@@ -11,7 +11,11 @@ import {
   LegacySpacer,
 } from '~/src/components/LegacyStack'
 
-# Stack
+# LegacyStack
+
+> **⚠️ Deprecation Warning**
+>
+> [Please use `Stack` instead.](?path=/docs/components-stack--docs) `LegacyStack` is deprecated. It is no longer supported for updates and may be removed in the next major version.
 
 ## Overview