Provide a better example for the PluginSidebar slotfill.

[ryanwelcher]

What?

This PR adds a more robust example for the PluginSidebar slotfill that demonstrates how to build out a basic UI.

Why?

This example is a more practical demonstration of how to use the components in the slotfill.

[github-actions]

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Co-authored-by: ryanwelcher <[email protected]>
Co-authored-by: juanmaguitar <[email protected]>

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[github-actions]

Flaky tests detected in 447d311.
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/10217289530
📝 Reported issues:

• [Flaky Test] Create a pattern with overrides #58009 in /test/e2e/specs/editor/various/pattern-overrides.spec.js