Fix GitHub-style callout rendering

The callout plugin wasn't working because:
1. Contentlayer cache was preventing the plugin from running
2. The plugin wasn't handling blockquotes with whitespace text nodes
3. The plugin needed to skip whitespace-only children to find actual content

Updated the rehype plugin to:
- Skip whitespace-only text nodes when looking for [!TYPE] markers
- Handle both direct text children and text within paragraphs
- Properly extract the callout type from regex match
- Clean up empty text nodes after removing markers

Now callouts render correctly with proper structure:
- Header with icon and title
- Content wrapper with styled box
- All 5 callout types supported (NOTE, TIP, IMPORTANT, WARNING, CAUTION)

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

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

contentlayer.config.ts

@@ -88,6 +88,7 @@ export default makeSource({
   markdown: {
     remarkPlugins: [remarkGfm],
     rehypePlugins: [
+      rehypeCallouts,
       [
         rehypePrettyCode,
         {
@@ -98,7 +99,6 @@ export default makeSource({
           keepBackground: false,
         },
       ],
-      rehypeCallouts,
       rehypeSlug,
       [rehypeAutolinkHeadings, { behavior: 'wrap' }],
       /**