Document createIslandSignal for sharing state across Solid islands

[matthewp]

Description

Documents the new createIslandSignal API added in withastro/astro#17159, which allows sharing reactive signals across multiple Solid islands on the same page.

Adds a "Sharing state across islands" subsection to the SolidJS integration guide with:

• How to create and pass a shared signal from .astro frontmatter
• How Solid components consume the signal with the standard accessor/setter pattern
• A note that signals can also be nested inside arrays and objects

[astrobot-houston]

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[ArmandPhilippot]

Thanks, Matthew! Except a missing <Since /> I think this looks good.

I was waiting before reviewing/approving because I noticed Chris comment on the implementation PR, and that's true we have a recipe for that. So, I was also curious to know why we need something specific for Solid!

[ArmandPhilippot]

We'll need a <Since /> here, I believe this should be the right version:

Suggested change

	### Sharing state across islands
	### Sharing state across islands
	<Since pkg="@astrojs/solid-js" v="7.1.0" />