CheckboxControl: Add custom label example to Storybook

[mirka]

Part of #51740

What?

In the Storybook for CheckboxControl, adds an example of how to accessibly implement custom labels/descriptions.

Why?

To help developers build custom implementations without breaking accessibility requirements.

Testing Instructions

See the "With Custom Label" story for CheckboxControl. The story description should be clear, and the example should meet the accessibility requirements for a checkbox.

Screenshots

[github-actions]

Flaky tests detected in 16e9d1e.
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/7710945383
📝 Reported issues:

• [Flaky Test] Overlay menu interactions in Safari (@webkit) #55816 in /test/e2e/specs/editor/blocks/navigation-frontend-interactivity.spec.js
• [Flaky Test] directives inside islands should not be hydrated twice #58332 in /test/e2e/specs/interactivity/tovdom-islands.spec.ts

[ciampo]

Great addition 🚀

[andrewhayward]

I know it would be a big breaking change, but it would be great if we could make label non-optional. There's no escaping bad labelling – it's rather hard to confirm that the label isn't an empty string, and impossible to determine if the label is actually useful – but we can at least ensure someone has to be a bit more intentional in not rendering a label.

⛔️ <CheckboxControl checked={ isChecked } />
✅ <CheckboxControl checked={ isChecked } label={ false } />

[mirka]

I know it would be a big breaking change, but it would be great if we could make label non-optional.

I'm definitely open to discussing more drastic improvements in #51740 as part of a package-wide, consistent label strategy for all our control components. It's just that there are still a ton of low-hanging fruit at the documentation level, which will be useful to put in place regardless of whether we make more drastic changes at the code level.