Add a new section to the SlotFill reference to show how to conditionally render Fills .

[ryanwelcher]

What?

This PR adds some examples to show how to conditionally render FIlls based on content. ie. Site editor vs post editor

Part of: #64749

[github-actions]

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

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[ndiego]

Suggested a minor tweak, but it looks great! 🚢

[github-actions]

Flaky tests detected in 25639bc.
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/10564990081
📝 Reported issues:

• [Flaky Test] should apply Heading block styles to all Heading blocks #49942 in /test/e2e/specs/site-editor/push-to-global-styles.spec.js