Add GitHub-style callout support

Implement proper GitHub-style callouts with beautiful styling:

Features:
- Custom rehype plugin to transform > [!NOTE] syntax
- Support for 5 callout types:
  * NOTE (blue, 📝)
  * TIP (green, 💡)
  * IMPORTANT (purple, ❗)
  * WARNING (orange, ⚠️)
  * CAUTION (red, 🚨)
- Gradient backgrounds with accent colors
- Full dark mode support
- Converts existing emoji callouts to proper format

Files:
- lib/rehype-callouts.ts: Custom plugin for parsing
- contentlayer.config.ts: Add plugin to pipeline
- styles/globals.css: Beautiful styling for all types
- content/: Convert 2 emoji callouts to [!TIP] format

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -35,6 +35,7 @@
     "rehype-autolink-headings": "^7.1.0",
     "rehype-pretty-code": "^0.14.1",
     "rehype-slug": "^6.0.0",
+    "remark-directive": "^4.0.0",
     "remark-gfm": "^4.0.1",
     "shiki": "^3.15.0",
     "tailwind-merge": "^3.4.0",