feat: add learn section from idempot-js

[mroderick]

Summary

• Import /learn/ section from idempot-js VitePress site to idempot.dev
• Add Mermaid diagram support using vitepress-mermaid-renderer (VitePress 2.x compatible)
• Create 5 educational pages about idempotency with framework context notes
• Update navigation to replace "Why Idempotency" with "Learn" section
• Remove old why-idempotency page (replaced by new learn section)

Changes

Infrastructure:

• Added vitepress-mermaid-renderer dependency for Mermaid diagram rendering
• Configured VitePress theme with Mermaid support (dark/light theme switching)

Content:

• Created docs/learn/index.md - Learn section overview
• Created docs/learn/why.md - Why idempotency matters (with mermaid diagrams, YouTube video)
• Created docs/learn/duplicated-vs-repeated.md - Duplicated vs repeated operations (with Hono framework context)
• Created docs/learn/client-key-strategies.md - Client key generation strategies
• Created docs/learn/spec.md - IETF spec compliance (with idempot-js implementation context)

Navigation:

• Updated top nav: "Why Idempotency" → "Learn"
• Converted sidebar from array to object format for path-specific sidebars
• Added /learn/ sidebar with 5 items
• Updated / sidebar to show "Specifications" and "Projects"

Cleanup:

• Removed old docs/why-idempotency.md page

Test Plan

• Dev server starts without errors
• All 5 learn pages accessible at /learn/*
• Mermaid diagrams render correctly in both dev and production
• YouTube video embed works
• Sidebar navigation works for all pages
• Top navigation "Learn" link works
• Internal links between learn pages work
• Code examples display with syntax highlighting
• Production build succeeds
• Production preview works correctly

Screenshots

The learn section includes:

• Mermaid sequence diagrams showing duplicate request problems and idempotency solutions
• Framework-specific code examples with context notes
• Comprehensive coverage of idempotency concepts

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Deploying website with    Cloudflare Pages

Latest commit:	3a26250
Status:	✅  Deploy successful!
Preview URL:	https://e9646b9f.website-33l.pages.dev
Branch Preview URL:	https://add-learn-section.website-33l.pages.dev

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