Theme docs: Update appearance-tools docs to reflect opt-in for backgroundSize and aspectRatio

[andrewserong]

What?

Add references to backgroundSize and a missing reference to aspectRatio to theme documentation related to appearanceTools.

Why?

With backgroundSize being included in WP 6.5 and being opted-in via appearanceTools, we should reflect this in the docs.

How?

Update the theme.json schema, and the two places in the docs that refer to appearance tools (I think there's only two places, but there could potentially be more).

Testing Instructions

Double-check spelling.

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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Co-authored-by: andrewserong <andrewserong@git.wordpress.org>
Co-authored-by: fabiankaegy <fabiankaegy@git.wordpress.org>

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[github-actions]

Flaky tests detected in 247e3fe.
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/7955166218
📝 Reported issues:

• [Flaky Test] default to a list of pages if there are no menus #47732 in /test/e2e/specs/editor/blocks/navigation.spec.js
• [Flaky Test] default to my only existing menu #59008 in /test/e2e/specs/editor/blocks/navigation.spec.js
• [Flaky Test] default to the only existing classic menu if there are no block menus #48033 in /test/e2e/specs/editor/blocks/navigation.spec.js

[fabiankaegy]

thanks for the update :)