Block Editor: Add documentation for SpacingSizesControl component

[Infinite-Null]

Related to: #22891
Closes: #52070

What?

Adds documentation for the SpacingSizesControl component by:

• Adding a comprehensive README.md file
• Adding JSDoc comments with proper syntax, examples, and prop descriptions

Why?

The SpacingSizesControl component currently lacks documentation, making it difficult for developers to understand and implement the component correctly:

• Clear documentation of all available props and their usage
• Examples showing how to implement the component properly
• Documentation of component views and internal structure

The documentation follows established patterns from other components while providing specific details relevant to SpacingSizesControl, making it easier for developers to use this component effectively in their blocks.

[github-actions]

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[github-actions]

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Co-authored-by: Infinite-Null <[email protected]>
Co-authored-by: glendaviesnz <[email protected]>
Co-authored-by: mburridge <[email protected]>

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