feat(reference): Update astro:assets reference for new Astro 6.0 behavior

[Princesseuh]

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In Astro 6, you can crop images and set fit and position without needing a layout. I'll admit, I don't fully understand how layouts work since I wasn't around when it was added, but hopefully I got it.

Related issues & labels (optional)

withastro/astro#14629

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[florian-lefebvre]

You need to update the upgrade guide as well, see #12322

[sarah11918]

So based on this, if fit and position no longer require layout, then that "Responsive Images Properties" section probably isn't needed anymore, and layout itself could just be thrown up higher into the full list of image properties too, eh?

Like, this isn't true anymore:

Setting the layout property on an <Image /> or <Picture /> component creates a responsive image and enables additional property settings.

That was the reason to have the section, so I'd suggest we just move the layout API reference up too, and remove that section.