DataViews: document combined fields

[oandregal]

What?

Improves documentation for the layout property of the view, and adds a section about "Combining fields".

Why?

It's undocumented.

How?

Before	After
Trunk	This branch

[github-actions]

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[github-actions]

Flaky tests detected in 281233a.
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/10615668568
📝 Reported issues:

• [Flaky Test] can be created by typing an asterisk in front of text of a paragraph block #48541 in /test/e2e/specs/editor/blocks/list.spec.js