[Layout foundations] Update `Box` component docs and guidance

[laurkim]

Update Box docs

• Description
• Examples: descriptions and visuals
• Add related components section

Update
Description	Box is the most primitive layout component. It’s a way to access Polaris design tokens.
Default description	By default, box has no padding, borders, background, or shadows, and border radius is set to zero.
Color description	Background color of box and text color inside a box can be adjusted using the Color tokens.
Border description	Border width can be adjusted using the Shape tokens, and a subset of tokens allows different types of border.
Border radius description	Border radius can be adjusted using the Shape tokens.
Padding description	Padding can be added to either all sides, left, right, top, and bottom, using the Spacing tokens.
Shadow description	Shadow can be applied using the Shadow token. The base is --p-shadow-base.
Related components	For more specific use cases, use the Card component.

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