[Modal border] Update Usage, Style, and A11y tabs

[tay1orjones]

Acceptance criteria

We need to update the images on the Modal component page/tabs to include the new modal border.

Usage tab

• Update all modal images to include the border.

Style tab

• Update all modal images to include the border.
• Color section: Add row item in Color table for the $border-subtle*
• Structure section: Add indication in image of 1px border in magenta. Also add this for the button divider. Add border specs for the container and button divider to the table.

A11y tab

• Update all modal images to include the border.

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Review

• Make PR
• Review
• Merge PR