Post title: Add example of the block

[MaggieCabrera]

What?

Add a basic example to the core/post-title block.

Part of #30029

Why?

Because it was showing empty before

How?

I define a viewport size for the example

Testing Instructions

Open the block inserter and search for the title block, hover over it and you'll see the example

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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Co-authored-by: MaggieCabrera <onemaggie@git.wordpress.org>
Co-authored-by: jameskoster <jameskoster@git.wordpress.org>

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[jameskoster]

Working for me :)