Refactor memoryArena website

[Raezil]

What changed

• rebuilt the landing page around a performance-focused visual system
• split presentation and behavior into maintainable local CSS and JavaScript assets
• added responsive navigation, accessibility improvements, and SEO/Open Graph metadata
• added an interactive arena allocation visualizer and copyable install command
• replaced the Chart.js runtime dependency with native CSS benchmark visuals
• tightened benchmark context, allocation-model guidance, and lifetime-safety messaging

Why

The existing page had strong technical content, but presentation, behavior, and charting lived in one 977-line document with a runtime CDN dependency. This refactor makes the site faster to load, easier to maintain, more distinctive, and clearer for Go developers evaluating the library.

Validation

• verified examples against the current MemoryArena, ConcurrentMemoryArena, and PooledMemoryArena implementations
• parsed the HTML successfully
• checked JavaScript syntax with Node
• confirmed the branch references only local CSS and JavaScript assets
• confirmed the branch is three commits ahead of main with no unrelated changes