Excerpt block: add example of the block

[MaggieCabrera]

What?

Add n example to the core/post-excerpt block.

Part of #30029

Why?

Because it was showing empty before

How?

By defining a viewport width size for the example

Testing Instructions

Testing Instructions for Keyboard

Create a post and add some content to it.
Open the block inserter and search for the excerpt block, hover over it and you'll see the example

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.