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>

contentlayer.config.ts

@@ -4,6 +4,7 @@ import rehypeSlug from 'rehype-slug';
 import rehypeAutolinkHeadings from 'rehype-autolink-headings';
 import remarkGfm from 'remark-gfm';
 import rehypePrettyCode from 'rehype-pretty-code';
+import { rehypeCallouts } from './lib/rehype-callouts';
 
 export const Post = defineDocumentType(() => ({
   name: 'Post',
@@ -97,6 +98,7 @@ export default makeSource({
           keepBackground: false,
         },
       ],
+      rehypeCallouts,
       rehypeSlug,
       [rehypeAutolinkHeadings, { behavior: 'wrap' }],
       /**