fix: Add figure-img component

Changes

• Update README
• Add <figure-img> component
• Update spacing on <figure> to margin
• Update <figure> usage to <figure-img>
• Remove custom usage of gl-p-5 class on <img> in <figure> to align image content to viewport width on mobile

Outcome

• Remove duplicate alt, aria-label, <figcaption> needing to be manually set
• Remove role="img" needing to be manually set
• Single source of truth for content images to update as aria/role="img" support improves for <figure>
• Consistent spacing
• Remove custom styles and compute max-width from width prop
• Remove img-50 etc. classes which reduce images on mobile to 50% of viewport width, too small for small screens

Screenshots

Before	After