adds: readme for block editor component spacing-sizes-control

[AkmElias]

What?

Add documentation for the spacing-sizes-control block editor component.

Why?

documentation/readme file was missing.

How?

This PR addresses issue #22891

Testing Instructions

1. Go to packages/block-editor/src/components/spacing-sizes-control and open the readme.md file.

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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Co-authored-by: AkmElias <[email protected]>

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[github-actions]

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