feat: migrate learning-hub articles into Astro website

- Add Astro Content Collection for learning-hub articles
- Move 5 fundamentals articles into website/src/content/learning-hub/
- Create ArticleLayout.astro for rendering markdown articles
- Create index page listing all articles in recommended reading order
- Create dynamic [slug].astro route for individual articles
- Add Learning Hub to main navigation and homepage cards
- Add article prose and index page CSS styles
- Update internal links to use website URLs

website/src/layouts/BaseLayout.astro

@@ -99,6 +99,10 @@ try {
               href={`${base}samples/`}
               class:list={[{ active: activeNav === "samples" }]}>Samples</a
             >
+            <a
+              href={`${base}learning-hub/`}
+              class:list={[{ active: activeNav === "learning-hub" }]}>Learning Hub</a
+            >
           </nav>
           <div class="header-actions">
             <button