Refactor navigation links to use Next.js routing and improve post handling

- Updated AboutButton to navigate to the about page using Next.js router.
- Changed HeaderButtons and MobileNav to link directly to the about page.
- Modified Home component to exclude the 'about' post from the posts list.
- Added a helper function to strip YAML frontmatter from post summaries.
- Enhanced API routes to handle reading and writing markdown files for posts.

src/app/HeaderButtons.tsx

@@ -33,7 +33,7 @@ export default function HeaderButtons() {
         />
       </a>
       <a 
-        href={typeof window !== 'undefined' ? window.location.origin.replace('3000', '80') : '#'} 
+        href="/posts/about" 
         target="_self" 
         rel="noopener noreferrer"
         className="focus:outline-none focus:ring-2 focus:ring-blue-500 focus:ring-offset-2 rounded"