refactor: update HorizontalLayout examples to support both Lumo and Aura

[vursen]

• Added theme switcher to all HorizontalLayout examples on main and styling pages
• Simplified spacing documentation: removed the spacing variants table and render-only demo, replaced with concise inline code showing custom gap values and CSS custom properties
• Deleted horizontal-layout-spacing-variants.ts demo that showcased Lumo-specific spacing variants
• Added reference links to base gap properties and Lumo space properties for spacing customization
• Increased the horizontal padding for .example-item to make the "Wrapping Horizontally Aligned Items" example render correctly with Aura