feat(branding): optimize logo display and prevent layout shift

Jim Birchrequested to merge
issue/diagnosis-3580079:3580079-branding-block-image into 1.x
  • Add logo customization documentation to README
  • Implement CSS aspect ratio preservation with min-width
  • Add performance attributes (loading="eager" fetchpriority="high")
  • Include SVG viewBox guidelines for optimal CLS scores
  • Reserve minimum space for logos to reduce layout shift

Closes #3580079

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