Composite: add context-forwarding with SlotFill example

[ciampo]

What?

Extracted from #64723

Add a new "With Slot Fill" Storybook example for the Composite component

Why?

To better showcase the utility of the Composite.Context component, especially as we're planning on obfuscating the type of the store prop on the context.

How?

• Added a storybook example
• Tweaked the example's shown code to include the custom slotfill creation

Testing Instructions

• Open the "With Slot Fill" Storybook example for the Composite component
• Interact with the sample, make sure it works as expected
• In the Composite "Docs" Storybook page, check the code snippet generated for this new example

Screenshots or screencast

[github-actions]

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Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: tyxla <tyxla@git.wordpress.org>

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[tyxla]

Looks great and tests well! 🚀

Thanks for building a story for this elaborate use case, @ciampo 🙌

[mirka]

Wow, good call to document this 🙇 Thank you.

Also TIL display: contents 😳

[ciampo]

Also TIL display: contents 😳

I did while working with DropdownMenuV2 and its sub-grid layout (which is IMO very cool!)

Wow, good call to document this 🙇 Thank you.

I wonder if we should have a more systematic approach to it:

• should all components that use context to "function" have dedicated SlotFIll docs and examples?
• as an alternative (or in addition to the above), should we move these docs and examples under the SlotFIll component?
• as an alternative (or in addition to the above), should we have a separate docs page on Storybook ? I feel like we should document this properly.

[mirka]

I wonder if we should have a more systematic approach to it

Yes, I think we could very well do all the things you listed! It's a tricky thing that could waste a lot of time to figure out.