docs: clarify UIManager LayoutAnimation scope and New Architecture behaviour.

[supunyasanthaofficial]

Fixes #889

Summary

Clarifies the use of UIManager.setLayoutAnimationEnabledExperimental in docs/layoutanimation.md:

• Clarifies that on Android with the Legacy Architecture, this flag enables layout animations globally and only needs to be called once (for example, in the app's root entry file index.js or root component) before triggering layout animations, rather than per component.

• Adds a note clarifying that on the New Architecture, layout animations work automatically and calling setLayoutAnimationEnabledExperimental is unnecessary (it is a no-op).

Test Plan

- Verified formatting and linting pass (`npx eslint docs/layoutanimation.md`).
- Verified documentation builds successfully with Docusaurus.

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[Simek]

Hello @supunyasanthaofficial, thanks for the update on this! 👍

Since you edited only the docs file the change would be visible only in "Next" docs and in future React Native versioned docs. If you want this change to apply to older, already present versioned docs, you would need to backport your changes:

• https://github.com/react/react-native-website#versioned-docs

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