Website Polish and Cleanup (pytorch#15272)

Code Quality Improvements

- Fixed HTML syntax error: Corrected invalid closing tag (</h1> →
</div>) on line 1179
- Removed duplicate code: Eliminated ~80 lines of duplicate CSS
definitions and redundant JavaScript event handlers
- Fixed JavaScript event handling: Added explicit event parameter to all
switcher functions (switchBackend, switchPlatform,
switchMultimodalPlatform) and updated all 11 onclick handlers for better
browser compatibility

Architecture Improvements

- Externalized CSS: Moved ~1000 lines of inline CSS to separate
style.css file, reducing HTML from ~2107 to ~1070 lines
- Removed deployment artifacts: Removed hardcoded Cloudflare analytics
script that should be injected at deployment time

Design Consistency

- Standardized section padding: Unified all section padding to 5rem 0
(except Hero at 7rem 0 4rem and Footer at 3rem 0)

Marketing Updates

- Updated backend names to marketing-approved branding:
  - XNNPACK with Arm Kleidi
  - Qualcomm® AI Engine for Qualcomm® Hexagon™ NPU
  - Apple Core ML for Apple silicon
  - NXP Semiconductors' eIQ® Neutron NPU
  - Apple Metal Performance Shaders (MPS)
  - OpenVINO from Intel
  - Samsung Exynos NPU
  - Arm Ethos-U NPU
  - and others

Author: mergennachin