Migrate site to Kumo design system

[mattzcarey]

The site currently uses custom components with Tailwind CSS. Migrate to Cloudflare's Kumo design system for consistency with other Cloudflare properties.

Tasks

• Add @cloudflare/kumo dependency
• Replace custom theme toggle with Kumo theme provider
• Replace custom CodeBlock with Kumo code block component
• Replace custom PillButton/PillLink with Kumo button components
• Replace custom typography styles with Kumo tokens
• Audit remaining custom components for Kumo equivalents