Section Styles: Update block style variation docs

[aaronrobertshaw]

What?

Updates global styles documentation around section styles a.k.a block style variations.

Why?

The existing docs were outdated and missing new features.

How?

• Removed incorrect outdated info
• Added overview of new block style variation features such as theme.json partials, and registering variation styles via register_block_style

Testing Instructions

1. Give the markdown changes a proofread
2. If you're really keen you could even follow the snippets in the updated docs to try adding some block style variations and styles to your current theme.

[github-actions]

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Co-authored-by: aaronrobertshaw <[email protected]>
Co-authored-by: ramonjd <[email protected]>

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[github-actions]

Flaky tests detected in aa6ffe5.
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/12427502186
📝 Reported issues:

• [Flaky Test] Navigates the items list via UP/DOWN arrow keys #64334 in /test/e2e/specs/site-editor/dataviews-list-layout-keyboard.spec.js

[ramonjd]

LGTM