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/ArticleLayout.astro

@@ -0,0 +1,47 @@
+---
+import BaseLayout from './BaseLayout.astro';
+
+interface Props {
+  title: string;
+  description?: string;
+  estimatedReadingTime?: string;
+  lastUpdated?: string;
+  tags?: string[];
+}
+
+const { title, description, estimatedReadingTime, lastUpdated, tags } = Astro.props;
+const base = import.meta.env.BASE_URL;
+---
+
+<BaseLayout title={title} description={description} activeNav="learning-hub">
+  <main id="main-content">
+    <div class="page-header">
+      <div class="container">
+        <nav class="breadcrumb" aria-label="Breadcrumb">
+          <a href={`${base}learning-hub/`}>← Learning Hub</a>
+        </nav>
+        <h1>{title}</h1>
+        {description && <p>{description}</p>}
+        <div class="article-meta">
+          {estimatedReadingTime && <span class="meta-item">📖 {estimatedReadingTime}</span>}
+          {lastUpdated && <span class="meta-item">Updated: {lastUpdated}</span>}
+        </div>
+        {tags && tags.length > 0 && (
+          <div class="article-tags">
+            {tags.map((tag) => (
+              <span class="tag">{tag}</span>
+            ))}
+          </div>
+        )}
+      </div>
+    </div>
+
+    <div class="page-content">
+      <div class="container">
+        <article class="article-content">
+          <slot />
+        </article>
+      </div>
+    </div>
+  </main>
+</BaseLayout>