Add alt text to images in Spacing page for screen reader accessibility

[Copilot]

This PR addresses an accessibility issue where images in the WinUI 3 Gallery Spacing page lacked alternative text, causing screen readers to announce them as generic "Image" elements without providing meaningful context to users with visual impairments.

Changes Made

Added AutomationProperties.Name attributes to two Image elements in SpacingPage.xaml:

• Cards layout image: "Example of spacing in a page with cards layout"
• Form layout image: "Example of spacing in a form layout"

Before

<Image Source="{ThemeResource CardsImage}" />
<Image Source="{ThemeResource DialogImage}" />

After

<Image Source="{ThemeResource CardsImage}" AutomationProperties.Name="Example of spacing in a page with cards layout" />
<Image Source="{ThemeResource DialogImage}" AutomationProperties.Name="Example of spacing in a form layout" />

Impact

• Accessibility: Screen readers will now announce descriptive context instead of just "Image"
• Compliance: Meets WCAG 2.1 success criterion 1.1.1 (Non-text Content) and MAS 1.1.1 requirements
• User Experience: Users with visual impairments can now understand what each layout example demonstrates
• No functional changes: This is purely an accessibility enhancement with no impact on existing functionality

The alt text is descriptive and contextual, helping users understand that these are visual examples of different spacing patterns used in UI layouts.

Fixes #1967.

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[niels9001]

/azp run

[marcelwgn]

/azp run