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/pages/index.astro

@@ -73,6 +73,13 @@ const base = import.meta.env.BASE_URL;
             </div>
             <div class="card-count" data-count="tools" aria-label="Tool count">-</div>
           </a>
+          <a href={`${base}learning-hub/`} class="card" id="card-learning-hub">
+            <div class="card-icon" aria-hidden="true">📚</div>
+            <div class="card-content">
+              <h3>Learning Hub</h3>
+              <p>Articles and guides to master GitHub Copilot</p>
+            </div>
+          </a>
         </div>
       </div>
     </section>