Composite: fix Storybook docgen

[ciampo]

What?

Refactor how Composite components are declared and exported to allow Storybook to generate better docs

Why?

Having Storybook automatically generate docs saves us time and ensures us that our docs are always aligned with the component's implementation.

How?

Inspired by @mirka 's #64654 (comment), the bulk of the change is:

• moved each subcomponent to a separate file;
• added named exports for each subcomponent.

On top of that, I've updated the contributing guidelines accordingly, assuming that this should be the recommended way of declaring / exporting sub-components.

Testing Instructions

• Make sure the project builds;
• Check Composite (V2) Storybook docs and examples, make sure that props (and their controls) work as expected, and that the examples also work;
• Smoke test usage in the editor — although no runtime changes are included in this PR, using the lock/unlock API means that what would normally be a build-time error, becomes a runtime error

[github-actions]

Warning: Type of PR label mismatch

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• Type-related labels to choose from: [Type] Automated Testing, [Type] Breaking Change, [Type] Bug, [Type] Build Tooling, [Type] Code Quality, [Type] Copy, [Type] Developer Documentation, [Type] Enhancement, [Type] Experimental, [Type] Feature, [Type] New API, [Type] Task, [Type] Technical Prototype, [Type] Performance, [Type] Project Management, [Type] Regression, [Type] Security, [Type] WP Core Ticket, Backport from WordPress Core.
• Labels found: [Type] Developer Documentation, [Type] Copy, [Package] Components.

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[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: mirka <0mirka00@git.wordpress.org>

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[mirka]

Ooh, really nice!

[github-actions]

Flaky tests detected in 72b49dc.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/10505201717
📝 Reported issues:

• [Flaky Test] Sorting #64430 in /test/e2e/specs/site-editor/new-templates-list.spec.js