Move iAPI docs from package to reference guides

[juanmaguitar]

What?

Move Interactivity docs from package @wordpress/interactivity to its own section under Reference Guides in the Block Editor Handbook

Why?

#60781

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Co-authored-by: juanmaguitar <juanmaguitar@git.wordpress.org>
Co-authored-by: fabiankaegy <fabiankaegy@git.wordpress.org>
Co-authored-by: ndiego <ndiego@git.wordpress.org>

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[fabiankaegy]

This makes sense now that it's in core 👍

[ndiego]

This is good to go. I'll create a separate PR to add a redirect from the old API reference doc to the new one. Once the redirect is in place, I will manually remove the old doc.